Whether or not the comma belongs in the first sentence depends on whether Now is intended as an adverb (look now rather than later) or as a mild interjection/interrupter. If the former is intended, delete the comma.
Change the comma after people to a period.
“Now, look at what you’ve done! You’ve ruined my favorite suit! You can’t just go around poking holes in people. It’s very impolite.”
(I freely admit that I like this particular example because it reminds me somewhat of a short paragraph in The Fallen, my twin’s second novel: “The man blinked and looked down at the blade embedded in his chest. ‘That was inexcusably rude.'”)