Killing someone is never OK, no matter what the circumstances are, but somethings don't need to be justified. If anda kill someone anda like/ is really close to someone anda care about to protect your loved one (which is what happened in both cases) it's acceptable, IMO at least.
I think both cases were the same. In both of them it wasn't necessary to kill the attacker (Vicki & Jess) but was done to assure the safety of everyone. anda can't justify one of them and not justify the other.
i guess because vicki was an evil vampire and wanted to kill elena and had to die for the humans of mystic falls but jesse only attackeed damon because he was made to but probably diddnt want to so people would have tried help him so really it wasnt his fault but it was totally vickis fault that she wanted to kill elena