Yeah the "stun" effect from munching on someone only works in a "masquerade" zone. As the museum is a "combat zone", any enemies there won't get stunned but attack immediately once you let go - so if you're going to munch on them, you pretty much have to kill them (or else run away while they shoot at you).

BTW you don't need to actually deactivate the lasers with the security computer - there should be a panel or something near the beams, if you bash that it should short them out. They won't turn off entirely, but they'll alternate in such a way that it's easily possible to sneak through without hitting them, with a bit of timing. Of course you won't need to do that now since you found another way - a way I didn't even know would work... ;)