Why would someone bully or troll others online, and then block them?

I get why trolls troll, but why do some trolls or cyber bullies antagonise people, and then when the people reply (nicely) to their comments, they act as though they are the victim and block the person they harassed?

Is there a particular reason for this? Psychologists?

Why would someone bully or troll others online, and then block them?
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