After a robery, a couple of days pass and nobody remembers... but a murder is a completely different story. I mean, I can beat all the guards in town with just a spear and a horse, but still going on a killing spree each time I visit a town is getting old

If your Stats for Speech and Charisma are high enough you can convince the guards that it was a accident or a secret mission but it's also tied to your reputation and your total known/remembered crimes.

I had a travelevent Bandits fighting guards and i killed one of the guards(by accident), gave up and used the charismaoption to convince them that it was exactly that..a accident in the heat of battle...they let me go and my reputation even raised normally for helping the guard

But if you killed the whole townguard it's unlikly it will work, even with high speech/charisma because every murder is remembered a looong time, together with your(most likly) low guard reputation...