The pun here is that 犯して (okashite - violate/rape me) is a partial homophone of お菓子 (okashi - candy). Thus the 'trick or treat' becomes 'trick or rape'.
On a strictly academic note, the pun also relies on the ambiguity of ください in that it can mean 'may I have' and 'would you please [do]'.
If you don't violate me I'll play a trick on you!Don't be ridiculous!Aren't you supposed to ask for candy?...Tch.I know that...
By the way you should stick your candy right here.
