I think "...assisted in genocide" as written implies intention pretty clearly, honestly. The letter writer is using the genocide, not Paul's inability to see what his father was up to, as the disqualifying attribute. A plain reading of "assisted in genocide" would suggest intention absent other qualifications that aren't in the letter.
As an aside, "[MJ] would not choose a man who assisted in genocide" in specific comparison to Peter is not a slam dunk if the letter writer meant to imply "unintentional deaths caused by intentions gone poorly" is obviously disqualifying - Venom and Carnage have a shocking body count, for example. And yes, that's fewer deaths than Paul might be culpable for, but as a statement about MJ's morality in regards to chosen partner, "he's partly responsible for thousands, not millions, of deaths" is ...something.