On the contrary, it goes against man's (and other life's) natural tendency to love himself, to want to keep existing, which is most selfish but fine and necessa

On the contrary, it goes against man's (and other life's) natural tendency to love himself, to want to keep existing, which is most selfish but fine and necessary. Suicide stems from being insufficiently selfish. The additional problem you're highlighting is that suicide is also an injury to the common good. That sort of thing is somewhat orthogonal to the selfish/altruist dimension though we conflate them a lot. Lastly, and this is probably more relevant to the history of it being wrong (at least in the west), it spits in the face of god.