Everybody has that thing that just scares the hell out of them.  For me, it's spiders.  There is no such thing as a good spider.  They should all die.  For others, clowns, snakes or bugs may induce panic.  For this lady, her nightmares will involve hissing cockroaches.

To be honest, this joke wasn't executed cleanly.  There wasn't a lot of thought that went into this prank in terms of where to place the bugs for her to find.  Personally, I would have put a cockroach in her bathroom drawer, in her hair dryer, by her toothbrush and any other place she might have found the little hissing bastards more naturally.

She knows she's being pranked before the first bug is even found.  You can't prank somebody by screaming, "I'm pranking you!!".  It just takes the fun out of it.  No, this wasn't a prank.  This was an assault.  There was not much funny about it after she found out it was a prank.  Had he just released the roaches without informing her she was being pranked, it would have gone much better.  This dude is just a douchebag.