Even if you don't hate Marner personally, it's pretty ridiculous not to "get" the hate by many fans. Have you not been paying attention?
Some fans have felt slighted & disrespected by Marner, going all the way back to the 2019 contract negotiation, where his camp clearly felt like they could use fan emotion as a bargaining tool.
As an extension of that, some fans find his level of play incommensurable relative to the contract, and the manner to which it was signed.
Some other fans can't stand his attitude / petulance, and just generally over-defensive demeanor. This, personally, is the subset of fans that I think are most unreasonable, but there also have been several examples where he just simply hasn't helped himself.