Given that the only ‘ain ma’alin’ I can think of off hand is a Meisis, it seems that the ruling is for everyone else’s benefit not his punishment.
On a completely unrelated line, the rationale could simply be that suicide is a chiyuv kares (RaMBaM Hilchos Aveilus. In general RaMBaM understands losing olam haba as kares see Hilchos Teshuva Ch. 2-3) and we are preventing him from doing the avera just like we would stop someone from eating chametz on Pesach.