When best man speeches go bad

I like a lot of other people have unfortunately witnessed some terrible wedding speeches in my time. This includes speeches from the father of the bride and groom to the matron of honour and the best man.
One terrible speech I have recently bared witness too was my friend from schools wedding. He had his brother as best man and the reason why his speech was excruciatingly painful to watch was the fact he was speaking about his brothers past relationships and how terrible his ex-girlfriends were for him. I suppose he had only the best intentions for the speech trying to include a funny anecdotal feature to the speech while also trying to say how wonderful his bride is in comparison to those 'dragon women' of his past.
Sadly however the bride and all the guests of the bride found it difficult to see his true intention. All they saw was a very embarrassing story for the bride and even more reason to think that her groom doesn't deserve such a woman as his bride. Like a chain reaction the whole audience's sentiment changed in an instant. Even us, the guests of the groom perspective changed from laughter to embarrassment after noticing the rest of the audiences response.
To say that after that best man speech the atmosphere was a bit cold would be an understatement. The grooms brother quickly sat down and the MC had a tough job laughing it off with a few 'segway' jokes before moving onto the other speeches.
For the rest of the night the mood was somber in comparison to the moments prior to the best man speech.
The best man holds a lot of importance to the audience as the best man is generally a longterm friend of the groom and holds the best position in providing credible insight of the groom.
Baring in mind some people being invited to the wedding may not of even met the groom before, but cast their judgement if him from opinions of others.
This is why the best man speech needs took be done right. If you are a Groom then you need your best man to understand this and even point him in the direction of a good best man speech template website. If you are a first-time best man then you need to treat this role seriously if not for you but for your best friends future married life.