PSG has the best position, and they are lucky because they have Lionel Messi as one of their big strengths. Therefore, when there are problems, usually Lionel Messi will be included because maybe there are also many who don't like Lionel Messi's game and try to make his enthusiasm at PSG collapse by receiving a lot of pressure like that.
Of course, the PSG fans think Messi isn't giving enough/his best for the club like he does when he is on International duty with Argentina, or when he played for Barcelona, and imo i think the fans are well within their rights to think that way. When they signed Messi there was quite a lot of optimism around the club, the fans thought he was the final piece of the puzzle that could finally deliver the CL for them, but two consecutive seasons now and they've been eliminated in the RO16 stage.
Having said that, i don't think it was a good idea for the fans to boo Messi a couple of weeks ago anyway, that's not the best way to show their frustration, thus i think the Argentine world champion is going to leave PSG this summer, he does not feel loved over there and so it is better for both parties if he goes. Anyway PSG's problem isn't with a single player, it is with the entire team.
Paris Saint Germain fans watched those magical moments of Lionel Messi in Barcelona shirt, they saw him single handedly tear opposing teams apart in his Barcelona days and they have also seen him win the titles for Argentina national team. And that's why they think he's not playing for the club the way he ought to.
Their frustration against Messi took a leap when Messi captained Argentina defeated France in last year's world cup final with Lionel Messi scoring two goals and also converting his own penalty during the shootouts. The good thing is that Barcelona is calling for his services at the moment so it's better he goes back to where he's loved and enjoy football once again