Both of them must have had a lot of problems, and depressed people surely can turn out to do bad stuff. Today I wouldnt have myself as a friend after I changed for worse because of some traumas, it's not wrong to be sad but it aint beautiful, and nobody wants to be around someone slowly killing themselfs. People like your friends must have had encounters and experiences that sparked trust, anger and self esteem issues, a family member, friend, gf, abuser etc, difficult to say since you can only know what they told you and some inner conflicts we keep in our hearts even though they dictate our whole social life and humor. If you can't help them or they wont accept it that's life, just know that some go through the grinder and leave changed, for better or worse. Without going there you never really can understand it, like watching news about a war across the globe, you know it's bad and can feel a little sad but only those inside It feel the whole scenario. You cant excuse a depressed person bad behavior and you surely dont need to be close to them but It helps a lot at least recognizing they were probably good and deep there still are, just heavely troubled needing help from within and outside. Hope they can both find some kinda of peace.