To devastate something is to ruin it, or to lay waste to it: to destroy it completely, leaving it like a vast, empty place.
If a hurricane devastates a town, it completely destroys it. If a swarm of grasshoppers devastates a field of corn, they totally ruin it. And if something devastates you, it destroys your heart: it makes you feel so sad that it's as if everything you love has been taken away from you. When you're devastated, you're so sad, so upset, or so disappointed that you feel totally empty inside.
This monster is devastating the building. He's smashing it to bits.
Here, George Michael feels devastated. Whatever just happened made him so sad and so empty inside that his heart is broken.