Hi, i just started a new game and i wonder if random killings on cities will happen if i don't start Dawnguard? Or i'm forced to delete the DLC from my hard drive? Thanks.

There is no way not to start Dawnguard. When you reach level 10 either a recruiter from the Dawnguard will approach you and ask you to join, or the guards will start talking about it. At that point the quest will be added to your journal.

Experiences vary on vampire raids. My own experience was that raids occur whether you visit Fort Dawnguard or not. Therefore I went with the option of deleting the DLC until I was ready to just do the questline straight through without visiting cities.