There are three types of negative comments you can get on your videos really:
Constructive criticism: Someone complains about something in your video, but it's a valid criticism. These comments are your friend and often it'll be your viewers/subscribers who point out things in your videos that need fixing (like poor audio quality etc) - so when you see these, don't let them sting, instead be happy and learn from them and your video quality will improve.
Valid but unhelpful criticism: Maybe you have a stutter, or do something people perceive as awkward, or have something odd about your looks etc that some people notice. Unfortunately, some people will decide to bring up these things even though there's nothing you can really do about them. If you want to grow in size on YouTube, be prepared for these things to get mentioned, and for your insecurities to get targeted by the select few who don't have good manners. It takes a certain tough skin if you ever take on any job/career that involves you being in the public eye under the scrutiny of 1000s of people - because all the worst things that someone could think to say about you, WILL be said at some point by someone. The best way to react to these kinds of comments if you're bothered by them, is to simply remove the comment so it doesn't start a conversation.
Unhelpful *and* invalid criticism: These are just the spammers/trolls intentionally trying to flame or bait you into a response. Again the solution is simple, delete any such comments you notice. Negative people spread their negativity in the hope of getting attention because people in that negative mindset tend to only be able to get enjoyment out of things like getting attention from others - so I never justify these comments with a response, as it gives them what they want.
So in short, if you get a negative comment, ask yourself: Is this something useful that I could learn from to improve my videos? If so, thank the person and apply it next time. If not, ignore them and/or delete the comment and move on.
