Has Windows 8 killed web games by not supporting plugins?

You don't necessarily need plugins to make web games. The best replacement for Flash for web games that I've seen so far is WebGL (check out Angry Birds on Chrome). The problem is IE9 doesn't have WebGL support, and neither will IE10.

So yes, I guess web games on IE are dead going forward, but it's only Microsoft's fault for rejecting WebGL. But I'm sure developers will continue to make WebGL games for 60%+ of the browser market (or more going forward) for Chrome, Firefox, Safari and Opera.

I think it will be more likely that this decision will kill any market share that IE has left.

I thought Windows 8 / Internet Explorer 10 was only removing plugin support in tablet mode. i.e. still supporting plugins (including Flash) in traditional desktop mode.