I think that it is a bad idea.
First of all, many rodenticides are designed with the risk of human consumption in mind. This means, they are made with a very bad taste for us humans, to prevent us from ingesting it. This already makes this as a method very uncomfortable, but alright, some people are able to look past taste...
The second thing is that liquid rat poison is often anticoagulant. This means, it makes it difficult for your blood to clot. This means, that any little tiny tear in your blood vessels (which are usually normal, unnoticed and fixed quickly by your body) will cause massive bleeding that will make you bleed out from the inside (or the outside if you have an injury). There's no real way to know where or how you will bleed. The blood vessels in your brain might start bleeding, causing brain damage, headaches, vomiting, whatever really. Your gastrointestinal tract might start bleeding, which isn't by itself lethal, but it can be painful, and you might poop and pee blood.
There are too many things that are dependant on randomness for rat poison to be a reliable method. While with SN it is clear that the inability of your blood to carry oxygen will make one (hopefully) peacefully pass out, what happens with rat poison is completely dependent on the kind of rat poison, and if it's the relatively common anticoagulant ones, it is just playing a painful lottery of whether you will suffer a lot.