It isn't tragic (as tragedy has a specific components), but it is pathetic and vile.
For what they did, the neighbors should not permitted to parent. Goodness knows, the US (and our species) has no need for the potential screw-ups they may do when raising their current child / children.
I can certainly understand the desire to put those two neighbors out of everyone's misery, though their actions do not any capital punishment; it warrants imprisonment for some time.
As to what kind of crime it is, I'd say that they enabled her suicide and that they effectively gaslighted Megan (Dictionary.com
under Webster notation, Wikipedia