This wasn’t done by Trump, it doesn’t help him for exactly the reason that you stated.
Trump has attacked at a personal level EVERY opponent as they got traction, Carson, Fiorina, Christie, Cruz (birther play, Rubio and now Cruz again.
I lived in the NY Metro area Trump has been doing this sort of stuff for 30 years. It is his SOP.
That may be the case, but this scenario has the potential to blow up in his face.
Let’s say Cruz has a good 30-40% support base at this time. Why make that support base hate your friggin’ guts forever?
You might succeed in taking Cruz out, but you won’t win the general.