Please do not (ever!) be mean when you reject a paper, or if you write a quick opinion. Harsh words are completely unnecessary. Just be professional, and imagine you are the one at the receiving end of the rejection letter. Be honest and direct, but always try to offer some constructive suggestions.
Should I be tough? No. Do not, in any way, write comments that can be construed as offensive. You should be an impartial, professional, honest and direct referee. So if things are missing, or if there are glaring mistakes, simply point them out in a plain way, and let the authors deal with the mistakes. If your comment is going to read like “the authors should know that…” then remove that comment and think of a way to point the problem out in a neutral way. — How to Referee a Math Paper.
Tags: reviewing