If he sees her kill someone in the Act 1 then there will be no suspense in the rest of the film as the fun could be in dropping obvious hints that the audience can see and the character ignores right up to a point where they are pulling their hair out frustrated by his denial. I’ll have a think about how to word his goal which is to try and stop her which, of course, he would do at the last possible minute and then way, way too late.