“Thou shalt not kill”, comes to mind.
Being sick is not a crime, so the death penalty exceptions do not apply. Being sick is not an act of war, so the soldier’s exceptions do not apply.
Again, thou shalt not kill; it applies to oneself, too.
Being mentally ill is not a crime, therefore there is no justification for killing oneself. Biblically, this is pretty straight forward.
You have it backwards. The church came first, and early Christians wrote the scriptures to clarify what the church taught.
The Scriptures, such as the letters of Saint Paul, only addressed theological issues that were controversial in the early church. No early Christian sect believed suicide or euthanasia were morally acceptable, therefore Saints Paul, Matthew, Luke, etc, had no need to specifically address these practices.
Contemporary documents address related topics. The mid 1st century document known as the Didache specifically condemns abortion for instance, even though the topic was not addressed documents that became scripture. While even the Didache may not specifically address euthanasia, it gives no hint that the practice would be accepted, either.
Basically, Scripture and other Christian documents served a specific teaching purpose. The mere absence of a condemnation should not imply acceptance. Humans are uniquely talented at creating vice; if scripture attempted to condemn every possible evil that humans might commit, it would be a thousand volumes long, and still incomplete.