As an atheist and someone who has studied the Bible as well as attended church in the past (not so much nowadays), I don't believe that suicide is a 'sin' specifically, contrary to what many modern Christians and preachers claim in today's world. I believe you are correct about the Catholic Church claiming suicide to be a sin and part of their reasoning from that time period is to use religion and fear to enact social order and conformity on the masses (general population of people rather than an act of worship).
In fact, as someone who has read the Bible before, I can claim that suicide is mentioned throughout many of the stories in the Bible (e.g. Judas after he betrayed Jesus, Samson, and Saul & the armor bearer, to name a few.). While I can't say that it is specifically a sin, those people certainly didn't go to Hell for simply CTB'ing, but for other reasons. As for the Judas story (Judas Iscariot), even if Christian scholars claim that he went to Hell, it would be more because he betrayed Jesus and committed blasphemy of the Holy Spirit (the unforgivable sin) rather than the fact he CTB'ed.
All in all, I don't believe in religious dogma or a God(s) (don't mistake this for claiming that there is no God(s); atheists only merely don't believe in God(s), but neither confirm nor deny that there is a God(s).), so I am not deterred by the prospect of going to Hell and suffering for all of eternity.