Is there a general meaning for 切る on the end of verbs?
I think it might have to do with "completely doing" something. If that's the case, how does ～切る on the end of verbs differ from ～てしまう, for example how do 成り切った and なってしまった "turned completely into" differ in meaning?
How does the meaning of 支え切れなくなる differ from 支えられなくなる "become unable to support"? Can 支え切れなくなる mean "will go past the point of being able to support"?
How does 疲れ切った differ from 疲れた "exhausted"? Does 完全に疲れた "completely exhausted" have a similar meaning to 疲れ切った?
Well, ～しまう kinda means that you ended up doing something, or that something happened that you don't feel so great about. I often use it at work to say something like そうすると、難しくなってしまう which gives the nuance that it's regrettable, but it will ended up being hard if you do it like that.
～きる means do something thoroughly. A common example is 本当にそう言い切れるのか？ Can you REALLY say that (and mean it/be sure about it)? Another one would be 使い切る which means use something until it's gone/used up.