1. When someone sympathizes with the aggressor in a conflict because that party is getting hurt more, disregarding that they (the "Tom") have actually started the fight by attacking the "Jerry" and therefore deserve what they get.
2. Imposing unreasonably high moral standards on someone defending themselves from a violent attack, often as an excuse to support the attacker.
"Man, I know Carlos was about to beat him up, but that little kid really shouldn't have broken Carlos' nose."
"Dude, Tom and Jerry Effect."
"I guess you're right."
"Damnit, why can't the Israelis just go home to Europe?"
"They've got a right to be there."
"Well then why do they have to kill so many innocent Arabs with such overwhelming force?"
"Because the Arabs attacked them first. Israel is the Jerry and Palestine is the Tom. Get over it."