Islam has a rather nebulous theology. Unlike the West with its more organized religions, seemingly all religion in Islam is local. Also, Christianity, while evangelical, doesn't command anyone to go out and convert those you can and kill the rest. How much emphasis is placed on this depends upon the local Imam who is interpreting it to his followers.
Christianity and Islam are two different religions. Despite the disdain I have for both, I much prefer living in the Christian sphere of influence. Christianity on the whole is far more tolerant than most Islam.
The theological answer, of course, is that suicide is a mortal sin, because only God can decide when one's life is over.
Suicide is a sin yes, but Jesus will forgive you for it. He forgives people who murders babies/children. Only thing that unforgivable is denying him. What a selfish guy Jesus is.
A psychological answer, since you reject the theological one, is that they want to be good for Jesus and be as free of sin as possible. Don't think for a minute that no Christian is sincere. They may believe in some falsehoods, but don't assume they aren't serious. Some aren't but many are.
You may think they are sincere, that is not the case. They are all scared of him. It's like a totalitarian rule where everyone obeys because they're literally scared to death of what would happen if they disobeyed their lord.
You don't know what you are talking about. You're just expressing so much hostility and contempt that you clearly can't (or won't) try to understand them. Most of my family is Christian and few of them fit the psychological straitjacket you describe. A lot of Christians (like many Hindus and Dervishes) experience ecstatic joy.
What's there not to understand when clearly this religion is a totalitarian rule. The number 1 commandment screams this sort of rule. Religious like to pretend the shortcomings/negative things do not exist in their system. I'm not sure why they feel the need to lie about it when we can all see what they're doing. Killing, hating, raping, and more killing.
You also overestimate how much many Christians are committed to their church's catechism. I knew a woman who told me how happy she was with her church. It wasn't a sect I was familiar with. "What do they believe?" I asked. "I don't really know or care," she replied, adding "I just like the people and the sense of fellowship there."
When I was young, I attended an Episcopal church with my family. That sect, more than most, is very undogmatic. My father still attends church regularly and really doesn't believe in Hell. Since "God is Love," He wouldn't send anyone to roast in a sea of flames. He will reveal himself to the departed soul who will then have faith and be allowed into Heaven.
While the most extreme Christians make the news, there are a lot of wishy-washy Christians who don't think a lot about theology.
Actually if I recall correctly, for Catholics suicide is the ultimate in sin. Due to that you can't ask for 'forgiveness" afterwards so you get sent directly to Hell for it and no collecting $200.
Catholics, like other Christians, take the details of theology with a grain of salt, assuming they even think about it. Birth control is murder according to Catholic theology, but a vast majority of Catholic women practice, or have practiced, artificial birth control
Every single Christian believes different things even if they are sincere - and I don't mean all the different sects teaching different doctrines (which is true too) but even within 1 single church there will be different views/beliefs.
Some people might think you won't be forgiven if you commit a sin knowing it's a sin and "counting on" being forgiven.
Some think you have to ask for forgiveness and repent while still alive on Earth after sinning.
Some might be specifically taught that all people who commit suicide go to hell.
Why would you ever want life saving medical treatment (other than pain management)?
Why would you ever want to save or be saved from dying?
I think I can answer both those questions with one anecdote..
I have a long-time friend who's a J.W. They, J.W.s, won't do any medical procedure which calls for them to receive someone else's blood nor will they donate blood. They are big on bloodless procedures. That being said, if she got into a car accident, needed a blood transfusion or she'd die and was in a position to accept or deny it.. She'd deny it. I know this because I asked her about it. She's has three girls. So I asked her "So you'd deny the transfusion and leave your girls without a mother?" .. She said "If that's what God's plan was, yes.." I was floored. So I asked,"So you'd essentially sacrifice your own life for a blood issue, thus leaving your kids without a mother and in doing so having them go to the very person you don't want them having contact with..?" (Note: She didn't want their father to see the girls..He had been abusive to my friend, their mother, which is why she left him..) .. She said,"If it was God's plan for that to happen, yes.."