Killing the ex husband makes no logical sense. He is as probably paying significant sums in both spousal support and child support. The only ‘logical’ motive would be to have full custody of the kids and then be free to move. That seems like a somewhat trivial excuse to kill someone who is paying you lots of money. Obviously this is 100% speculative and who knows what other internal disputes they had going on, just thinking out loud.
Divorce settlements commonly include a large life insurance policy that secures future alimony and child support payments with a lump sum payout if the ex-husband dies. Also, the children (and often the ex-wife) also usually inherit from the estate — and an executive career at Microsoft ought to have resulted in a substantial estate. That makes for a considerable financial incentive to off the ex-husband.