How come he's responsible for everything good Barcelona ever does but never responsible when they stop doing it?
He's not responsible for everything good Barcelona ever does, but 99% of the time, he is. Same for when things dont go Barcelona's way, some times its because of him, but most of the time they arent.
How do we come up with this conclusion? By actually watching the games without having an agenda.
Here is an example of Barcelona winning a huge game without Messi being the main responsible: vs PSG on 2017. It was basically Neymar, with the rest of the team, including Messi playing a very good collective game. But that was all Neymar, and no "Messi fanboy" will tell you otherwise, because its pretty obvious.
Now, an example of Barcelona losing a huge game with Messi being one of the main responsibles: vs Roma last year. He dissapeared the entire game and it was just a horrible Barca game all over the pitch. No "Messi fanboy" will deny that.
Those are exceptions to the rule though. Someone that watches Messi week in and week out will tell you that he's the main reason Barca win so much, because that is obvious if you actually watch the games. Hell, only take a look at the statistics and it will tell you the same story. Barcelona is basically relying on Messi to have a good day and most of the time, he does.
What happened against Liverpool is a perfect example of Messi taking the blame when he was by far the best player out of the two teams in both legs together. 10/10 performance by whoscored.com on the first leg, and 7.2 on the second leg. On average he played way better than anybody else. On the second leg he was Barca's best player by far, only Vidal came close who had an amazing first half followed by a shit second half.
This is the reason why you could ask any Barcelona fan and they will tell you that Messi is the least to blame. You will never every hear a single whistle at Camp Nou directed at Messi. Next game in La Liga if Messi plays, i can asure you his name will be chanted, because every Barca fan knows that Messi is 90% of the reason why Barcelona got as close to the trebie as they did, both last season and this one.
They know this because they actually watch the games. They actually saw Ter Stegen's and the defense's mistake ont he 4th goal, they saw Alba's mistake on the first goal, they saw the Barcelona forwards missing literally every chance except for Messi, etc.
Its as simple as that. People just dont say "oh its never Messi's fault". People do say its his fault, when it actually is, the thing is that he is so good that when his team loses, believe it or not, one or few of the other 10 players are more at fault than him.