I worded the first paragraph pretty poorly, but the point was that the players he had at the time should've been winning most of their aim duels hence being able to just "grab an AK and fak zem". Since players like NiKo were terrible in the major final, it's not just Karrigan at fault for the loss. However bad a strat was, players like Rain, Olof, NiKo, and Guardian should've been able to turn the game with individual skill, just look at Guardian on the mirage game as an example, or even Rain's insane CT Dust 2 games.
If I remember correctly, Karrigan did better than NiKo in 1 of the 3 maps and similarly over the whole series (bar inferno where NiKo woke up but rain dropped off the face of a cliff), and NiKo is meant to be the superstar carry potential player.
Also, NiKo was questioning calls as Karrigan made them in game and suggesting other things shortly after Boston. You never to that to an IGL under any circumstances unless you want to totally ruin them.
First off, it's plain disrespectful to the person who is leading your team to success as it makes you seem like you think you know better, which is absolutely not the case because NiKo is one of the worst IGL's to ever touch the game.
It also starts throwing team chemistry into question, with NiKo departing FaZe, no teammates said a single good word about him, the best he got was from Rain who said 'It will be... different playing without NiKo because I've heard him for so many years'.
If you do this to the person calling the strats, it causes the IGL to start doubting themselves and get in their own head, which is the worst thing that can happen as they'll constantly second guess themselves resulting in self sabotage.
The reason Karrigan became so bad at leading in the end of FaZe is because he had 'no support from his team or coach at all' (Quote from Guardian in a rivalry.gg teammates video, who also said the team totally failed Karrigan)
TLDR - NiKo destroyed FaZe, ruined Karrigan's confidence, and lost Boston.
Also I can try and link anything you want proof on.