The issue with Matt holding the Veto is that, as the Temptation nominee, if he pulls himself down, there’s no replacement for him.
So if Matt is on board to backdoor Cody, he has to use the Veto on someone else.
That means, if he pulls himself down, either Elena or Jason will be going home.
What’s particularly odd about this situation is, if Matt really is planning on using his Veto to take someone else down, why did he keep it in the first place?