You don’t need to “prove” or “justify” your disagreeing with him, you just need to question whether it’s worth responding to it with a non-comment (eg an emoji) and/or a downvote. Traditionally, downvotes are best used not as a button for disagreeing, but whether people respectfully contribute to discussion. I’d say a well-worded post that brings up an interesting point of debate, even if you disagree as a matter of opinion with their conclusion, is worth upvoting. If you don’t have anything to add, you don’t have to comment.

All this would matter more for discussing, say, politics, than whether a game was good, so it’s just a general tip of mine.