Talk about a grudge

I've read about many true crime cases and I've never seen anything remotely like this one. A guy gets fired by 2 professors in 2001, goes on to get a medical degree, and comes back in 2008 and 2013 to murder 4 people over the firing. The 2008 murders were the young son and housekeeper of one of the professors. The 2013 murders were of the second professor and his wife.

https://abclocal.go.com/wls/story?section=news/iteam&id=9174545

So, what's the problem?