If Set wasn't a pacifist, then could he kill someone by using Solid State Shield?
In an if-this-was-real-life sense, and not in a supported-by-the-game-engine sense: yes. If the target was asleep, unarmed, or otherwise unable to break the shield in time, and Set managed to make it airtight, he could suffocate someone with one.
Why is it that flash games always seem to use vector animation instead of frame by frame animation? Does it cause a lot of lag, or is it just too much effort?
Frame by frame animation is way, way more time-consuming (and difficult) to create than vector tween animations. Tweening is a way to get a game's animations made quickly and cheaply.