Sherlock may have exaggerated some parts of his best man speech, but mostly, it was entirely true. (He had to exaggerate some parts, otherwise people would think the whole thing was true, and he simply couldn't expose that much of himself, not to all those people, not when he was already revealing so much that he felt naked in his suit.) The things he said about John were all entirely true.
"Iwill solve your murder, but it takes John Watson to save your life. Trust me on that, I should know. He's saved mine so many times, and in so many ways."
See, they may have thought that part was an exaggeration, but it wasn't. People wouldn't believe it if he listed them off, all the ways and methods and attempts and successes that John Watson had saved Sherlock Holmes.
The cabbie, obviously, shooting him before Sherlock could take the (possibly poisoned) pill. At Soo Lin's flat, when he yelled through the letter box that he was Sherlock Holmes, distracting the assassin. When the Golem surrounded him in the flashing lights and skipping planets, and John held a gun up and threatened to shoot him if he didn't let Sherlock go. When John, clad in a bomb vest and an earpiece and a coat that wasn't his, jumped Moriarty and told Sherlock to run. When The Woman drugged him and left him on the floor, spinning and flailing, and John carried him home and tucked him into bed and oh he had a date that night but he stayed and kept him safe. Being there on danger nights when his mind wore bloody circles as it paced madly round and his skin itched with the memory of the needle. Shooting the hound that did exist when his eyes deceived him and he tried to push John away out of his own fear. All those days and nights of torture, with one all consuming thought on his mind home to John London Baker Street home to John home with John that kept him going. Through all the pain, the loneliness, the two years of suffering, that thought kept him going. (And even after the wedding, the ones he didn't yet know of, all the ways that could still come for John to save him.) Lying in the depths of his mind palace, a chained and manic Moriarty hovering above him, John whispered to him we're losing you Sherlock and he held on. When he was tubed and wired and nearly gone, but he was reminded that John Watson is definitely in danger and that was all he needed to make that final push back up.
You. You keep me right.
So the one time he saved John (murder murder murder) he felt he was barely scratching the surface.
There was nothing Sherlock could do to repay John. But by god, he could try. Because it was Mary, it was for Mary who meant everything to him, Mary and their unborn child, and Sherlock understood that. So he made sure they would be safe.
It was the least he could do.
He held the gun up to the man's head, in front of his brother and John and the universe and pulled the trigger. All for John Watson.
(When John murdered for him, they walked away giggling. When he murdered for John he ended up in tears. Had he really gotten so sentimental?)
All for John Watson, he told himself.
So what if he was sentimental.
Because it was John.