Yup, I mentioned the same thing some pages ago, while playing devil's advocate.
It was perfectly in character, once Magneto had control over the sentinels (which he would not have unless he was aware of them before they were activated, ergo, previous time-line where war DID happen), why stop there? He's always wanted a war between mutants and humans because he knows mutants would win, what better way than to start it turning humans' own weapons against them? That incidentally were created to kill mutants.
The pivotal factor in the future war that could end up in human and mutant deaths alike were the sentinels, but they were no longer a liability when Magneto took control over them, so he had no need of just shutting down the sentinel program when he could destabilize the entire country's government with his little play.
That ironic act of war, the stadium barrier, and the last presidential assassination on live TV seem very logical to me and within the confines of the Magneto character.
So again, yeah sure, he could have spoiled Trask's little party, mutants would still be treated like **** despite that, so the situation had potential to be more, so why stop there? And why does that make him an idiot?