Funny. My instincts are telling me that there's a Utility Monster behind that bush.

I'm not satisfied with the lifeist or the anti-deathist reasoning here as you present them, since both measure (i.e. life-count) and negadeaths as dominant terms in a utility equation lead pretty quickly to some pretty perverse conclusions. Nor do I give much credence to the boxed subject's own opinion; preference utilitarianism works well as a way of gauging consequences against each other, but it's a lousy measure of scalar utility.

Presuming that the box's inhabitant would lead a highly fun-theoretically positive fifteen minutes of life by any standards we choose to adopt, though, pressing the button seems to be neutral or positive (neutral with respect to my own causal universe, positive relative to the short-lived branch Omega's creating) -- with the proviso that Omega may be acting unethically by garbage-collecting the boxed subject when it has the power not to.