So I'm building my PC and touched the top of my i5 2500k a few times and got some fingerprints on it and shortly after I wiped it off with a tissue. I then applied the heatsink onto it. Well later I found out that your supposed to clean it using 70% alchohol or something and I didn't do that. Is my CPU going to be in trouble? Do I need to remove the heat sink and go buy more thermal paste to reattach it?
Yes, it is definitely in trouble. It can heat up so much that you're gonna have a potential nuclear meltdown on your hands.
Generally speaking, you'll be fine. Alcohol is a better solvent to swipe away any oil or old thermal paste. Finger prints have a little bit oil, but nothing really to worry about. There will not be any noticeable difference in temps.