To review a paper for a journal, or a research proposal for an agency, are adult undertakings. Yet, most of us act irresponsibly. Too often, our behavior resembles that of unruly kids who enjoy hazing someone new to the block, who relish pushing the head of a colleague underwater. In so doing, true, they may trip up a competitor. However, the bigger damage is to themselves and to their reputation.
But should you answer the call review a manuscript, in the first place ? If you have a vested interest, pro or con, disqualify yourself.
Read the submitted manuscript carefully and annotate it with a pencil. Do it on the day of receipt : « do unto others etc. » I would recommend that you draft your report on the same day, that you let it sit for a couple of days prior to returning to the manuscript for additional scrutiny a couple of days later. You will be surprised yourself by how much this small delay will improve your text — not to mention the gain to your mastery in your field.