1. Moving out is not easy
You are bound by the lease you sign unless your landlord lets you out of it. So if your roommate creates an unlivable situation requiring you to move out, be prepared to continue paying the lease until the end of its tenure.
2. You and your stuff are vulnerable
Beyond the potential of physical violence, there's the ever present threat of theft or damage to your property.
3. Lease terms alter
If you bring in a roommate to live with you your lease terms may change. If the roommate moves out, your lease may be invalidated. Know your lease agreement front to back.
4. Eviction is very difficult
Trying to evict your roommate can be more hassle than it's worth. Depending on the situation, you may be required to prove wrongdoing to authorities, or to persuade the landlord to kick them out.
5. Credit Rating
As a worst case scenario, if your roommate moves out, leaving you to pay the entire rent and you cannot pay the full amount and end up leaving early or evicted, you can be held liable and sued by the landlord, which can then result in a huge blow to your credit score and a massive black mark on your renting history, making future leasing near impossible.