How to deal with crappy flatmates.
In life, you will always encounter people who do not like you. Unfortunately, you can do nothing to improve them; you can only do things that make them less unbearable for yourself.
For example, a neat freak who gets extremely bothered about a few crumbs on the countertop—I'm talking EXTREMELY bothered. In this situation, you can only clean up after yourself as much as possible. However, if this isn't enough to get them off your back, here are some tips!
1. Communicate - People sometimes think that the only way to get your point across is to shout and scream, but this doesn't have to be the only way. Suppose you try to sit your flatmate down for a chat and calmly explain your expectations. Perhaps you could come to a middle ground. If the screaming starts to ensue, try implementing a talking stick. It feels very similar to primary school, but surprisingly, uni students respect the talking stick more than their flatmates!
2. Ignore them - A lot of the time, ignoring someone will make it better as they get bored of picking on you or doing things that make you want to punch them in the face. However, this option could backfire as it could come across as you're ignoring them because you're being petty; this is a tricky situation to handle as it could paint you out to be that bad guy, especially if the other person is an A star manipulator.
3. Talk to someone else - if you have a building manager or a close friend you can confide in, try telling them as much as you can about what the other person is doing. Some see this as 'snitching'; I see it as gathering a paper trail of evidence, so if it all eventually gets to be too much for you to handle, you have records of other things they have done, and they may be told to back down and be civil.
Obviously, if your flatmate won't change or at least try to listen to reason, then there isn't much else you can do other than co-exist and frantically look for second-year housing so that you know you won't have to live with them next year, too.