Either the study is valid or it isn’t. Disliking the authors should not be a factor.
Peer review means that a paper isn't obviously wrong or obviously useless, which isn't the same as meaning that it's right and useful. It's just the next step in the collaborative process of science.
It's possible for peer review to be tainted, but that's not necessary for incorrect results to get published. It could be anything from a simple mistake to outright fraud on the part of researchers.
Yes, any citations of the paper need to be questioned, though in many cases it will only be one piece of support of many, and won't change the conclusion. That's how science works; it's a continuously updated process. That's how we go from the unknown to the known; if we learned it all at once we'd have finished science a long time ago.