GasparZizou
Super Freak
Magneto didn't come off as stupid, there was more to it then just starting a war, it was to make a statement for all the mutants who were in hiding, and ashamed of who they were. Magneto broadcasting everything was also to inspire all the mutants around the world, show them they don't have to live in the shadow of H0m0-sapiens, that they are the next step in evolution, start an uprising.
Magneto's plan was far more brilliant than framing Trask. Mutants are going to start rebelling because of Magneto, and some might even join his Brotherhood, and he'll have the sentinels to control.
Trask stated that they needed more than just that DNA sample from Raven to continue on the project. In the original timeline, she is captured and tortured. So the future Sentinels still weren't a possibility until she's captured.
Also remember, Magneto knows where the sentinels were being manufactured, the program would not be able to continue after he murders Trask because Magneto would probably go back there and either destroy the place or kill everyone working there.
EDIT: just saw Kara explained it perfectly.
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Ah, I was gonna post that last part too :
Btw I convinced my friend partly because she had a cold and had a headache so she didn't want to debate she did acknowledge that Magneto wasn't all that stupid, giving Mystique's blood to Trask was still a pretty stupid mistake.