Its way more primitive than that - christian moral code prohibits suicide as those will end up in hell. Then not-so-small part of population feels entitled to project their own righteous path unto rest of humanity while even feeling great about doing that 'in accordance to god's wishes'. Next thing you know, any abortion is illegal, heck in places like Poland even basic human rights are violated by exactly this kind of self-righteous crowd.

Obviously this is just one of example of one random major religion and tells us mostly about how small many humans are and how it manifests across society. Feel free to apply this to much of evil caused by mankind, basically since we were humans.

Eye opening for me was when I met this fellow Texan guy in Nepal in 2008, great guy otherwise, who was telling me how all those arabs in Iraq and Afghanistan would make great christians. They just need to understand the right choice. He really believed it. If felt so surreal.

Interestingly, place like Switzerland which is highly religious has been for a long time bastion of self-assisted suicide, so that folks come here from all over the world to end their life peacefully. Tells you about how remarkably tolerant society this is (there are tons of other examples I can see every day around me, but that's for another topic).

To better understand people of differing beliefs, you have to temporarily pretend to accept their fundamental tenets.

To use your example, understand that most sects of Christianity teach that in order to gain entrance to heaven, one must accept Jesus as one's savior and at least try to follow the moral code promulgated by Christianity. People who do not will go to hell and suffer an eternity of torment. Understand, this is not a theoretical "what if" to them, they believe this to be true. Completely.

Thus, if you are a good person, would you not want to save others from eternal damnation? I mean, what kind of heartless human being would just let others go to hell without even trying a little? A good person, a kind person would therefore try to convert others to the path that would lead them to heaven.

Or similarly, if you believe a fetus is a human being then abortion is murder. Not the "termination of a fetus", but the literal equivalent of chopping up a newborn baby. If you understand that some people actually believe this, it is easy to understand why most pro-life arguments just seem facile to them. To them, those arguments are simply attempting to justifying the literal murder of innocent babies.

Putting yourself into the shoes of others and clearly understanding (not accepting as true, just accepting that they believe it is true) their underlying reasons is, IMO, critical to understanding what may, to you, be "crazy" or even "evil" positions.

Outlawing suicide is a lot older than Christianity. Aristotle was writing that suicide is an act of injustice toward state. And most probably he was not the first one to state that.