Personal experience is that people are both good and bad at the same time depending on what our presence or circumstances brings out in them.
Hitler had a lover.
Gangsters who kill for money or fun or to eliminate enemies have families.
Our army heroes are other countries worst enemies (and vice-a-versa).
I personally know a friend in India who everyone thinks is an arrogant asshole but when we get together, we have the most honest conversations and he will never sugar coat or tell me what I want to hear but tells me what he believes is TRUTH. This many view as 'bad/arrogance' and I think it is Good (rare one at that)...
Say for example, you are a generally good person but I just smack you in the face for no reason, will your MASK of good come off! I believe it will but that doesn't mean behind the mask you were a bad person - you may just be a neutral guy but I instigated that evil/bad side in you by putting you in that situation!