I believe wholeheartedly that people reap what they sow in this world and in the hereafter. Us mere mortals will not always see or be privvy to Allah's "adal" or justice - but those who have ill-will towards others rarely know "taskeen-e-dil" themselves. While I am far, far far from perfect, this belief makes be very careful about how I interact with others - lest I hurt them and to the best of my ability, while I may hurt someone, I'd like to think it was done with a lack of malice.
On a related note (and I know it's much easier said than done) I also believe that to seek direct justice is wrong and to wish specific punishment for someone else's misdeeds has negative consequences for us. Instead, we should ask for Allah's justice and leave it to Allah to determine what is deserved (or not). If we leave it to Allah, we will be rewarded for our sabar and aqeedah.