In principle, no, they don't have to suffer. In practice, probably. And the reason for that second one is... well basically it goes down to the deepest fundamentals of things. What... is this place? We don't know a lot, so we can put aside speculations, but what we do have a pretty good idea of is this place is one where things function a certain way. We can call that "physics" (the rules of the functioning of this place). And what that suggests is that stuff(small various particles that make up bigger stuff) moves around in a certain way, relating to energy, matter, forces like gravity(big stuff impinges its power forcefully on small stuff), entropy(stuff breaks down over time, order breaks down into disorder), right? Really really big place, lots of particles following rules.
What the hell does that have anything to do with the question?
Okay so... within that space... special kinds of particles make up stuff that organizes itself and then replicates itself. We can call that special stuff biology, and it does this by using fuel. Sometimes it survives, sometimes it dies. That's one problem, since there's no infinite source of fuel. So it's scarce to a degree. Another problem is that now you have these things that are competing. Some special stuff even eats the other special stuff to make it die, and uses it as fuel. As this whole project gets more and more advanced over hundreds of millions of years, what decides which of these things actually win the competition? Well... how good they are at accessing the fuel, and how good they are at surviving the game called "acquire the fuel, make more copies of yourself".
What that opens up is, the field of strategy. Which strategies are best for winning? They can't just all be good right? There's a fact of the matter. Once you're there, we can go on forever about small details, but the only important big picture fact is this:
It's all just about power.
Not everything can be equally powerful. That would make the idea have no meaning. Because the competing things have these scripts called genes, they become expressions of strategy. So it's like a video game where you have a bunch of little monsters that fight to the death with different abilities, and you can see at the end of the bloodbath, which ones "win". Then you just repeat it over, and over. Except unlike a game, these are conscious things that can suffer.
Once you get to the question of power, there's an interesting fact about it in that it tends to be a feedback loop. The more you have, the more you express it. The more you express it, they more you have. And so on, and so on.
(sometimes expressing power isn't intuitive-- part of power can mean being invisible, because if you're invisible, no one can destroy what they can't see-- that is power)
It's a lot like gravity if you think about it. The more gravity a black hole has, the more smaller stuff it suck in from a greater range, which increases its mass, which... increases its gravitation pull, and so on, and so on, until it devours the entire universe.
And that is basically why suffering exists, because the meaningful part of the universe(not like... the rocks and dust in space, but like the actual beings and creatures that can get tortured-- that's the part that matters) is composed of lots of beings that all just want to not suffer, but they're all in this mentally ill game of being wired to seek power, and avoid falling victim to the power of others, in a way that is not a mutual or social game at its root even if there are some appearances that can seem win-win or pro-social.
But remember the entropy thing? That seems so disconnected from this human level we're now talking about. But here it is at the human level.
(Tangent first: Evil is basically just a strategy to express power, because it fucks up peoples lives and then says, "Your move". It's inherently a power move unless it's punished somehow and often times the punishment doesn't even fix anything. Who cares if the person who killed the only person you love is now in prison? The worse scenario is, it can't truly be punished because power eventually has a ceiling where there is nothing *more* powerful at the level above-- there's a top level of the heirarchy and that is also where the seat of evil sits. Because it cannot be punished, it can do whatever it wants or it can justify. It's not that humans long ago were stupid, they understood this, and historically have masked this problem by inventing the idea of a God who is perfectly powerful but also perfectly fair and loving).
Okay... entropy. Stuff breaks down from order into disorder if we look at some boring atomic stuff, what does this have to do with evil or power or why we have to suffer?
So imagine you're in a village with a next door neighbor and you both have these nice little cottages you built. Your neighbor doesn't like you because they just lack any kind of good intention and they're completely selfish and envious and resentful and so they've justified that you deserve to be harmed. This neighbor needs almost no effort to wait for you to be gone, light a match, throw it onto your nice little cottage and have it burnt to ashes when you return. That is because the "wind" of entropy, is pushing at the back of evil(aka power). It naturally uplifts the behavior. You can imagine it like competing on a race track but a special wind is pushing your opponent forwards but not you.
The same way, you are now screwed. You have to work downwind from entropy, you have the very difficult task of rebuilding your cottage. Compare how much effort it is to fix this situation(create order) to the effort your evil neighbor used to destroy this situation(disorder). While you're busy, your neighbor can run for mayor of the village to continue whatever megalomaniacal bullshit they are interested in since they are wired to seek power(this is a function of their genetics, which are just a special arrangement of the small particles we talked about in the beginning). You're not the one plotting and scheming to rule the village, you just wanted to enjoy your life in your cottage. But now you can't-- you're going to suffer the consequences of the physics, the DNA based evolution, and the game theory of the universe.
This turned out to be a long post but there's one more very important detail to this story. Someone may think, "Okay but... surely now things are better. We've made progress. Now it's not cottages, but we can have things like security cameras, so when the evil neighbor tries to burn our house down, we go to the police with the footage, and they get punished, and slowly but surely things will keep getting better." That... is ... unfortunately, I deeply regret to say, the opposite of what happens. What actually happens is, the same way that the organisms who couldn't survive the "win the fuel and reproduce" game died off, the organisms who couldn't survive the "be evil pieces of shit and get away with it" die off. That does not mean evil dies off with time, that just means evil only gets stronger and stronger and more and more masked and camouflaged with time.
That's why people suffer at the expense of others. If this problem was completely solved, it would just be an upwards motion into a better and better world because humans are more than intelligent enough to create genuine improvements. But since this problem cannot be solved (or at least it hasn't even been identified by anyone in any clear way to be solved), the only progress you will see on Earth is a kind of superficial illusion that acts as a mask for malevolent and power obsessed beings to function undetected. The real sinister truth is that the "problem" you're seeing and would hope to solve, is actually not a bug to those who rule reality, it is a feature that they feel is "the way forward" towards greater and greater power.