There are catches in psychology. For one, what type of psychologist? Counseling, therapist, child, developmental, organizational (business-based, managerial or even human resources), social, research or something else? Many people hear "psychologist" and automatically go to therapy--yet advertisers and advertising is a branch (in a sense) of psychology. News casters even use psychology. Parents use psychology daily when they have a child.
1) Psychology runs tests and experiments to see what happens.
2) Psychology has contradictory results, they are expected (at times) and influence new tests and ways of thinking (psychology evolves, it has to, as cultures evolve as well).
3) Psychology is against religion and free will--well not exactly, but it explains and shows how people act the same or similar ways with certain conditions, and how people can be influenced without much effort. Based on this, blaming people for their actions, or even holding them completely responsible for their actions makes little sense (although prisons make sense from the point of view of protecting the public from murders and such).
4) Psychology is anti-capatilism --again not exactly, but as within psychology learning that environmental and social situations can help, hinder, or cause problems for people, a system of "rich" and "poor" and where people need to work (destroying family time) to pay bills and eat damages the person and chances of success.
But for why they are hated long version (in general)? Probably a few reasons:
1) They tell people what research shows--and depending on the conditions of research different results can be seen (ex, a nice caring person may torture someone given the right circumstances, and then never torture again [nor think of the torture as torture]). Opposing results in psychology is not only common and expected, but also leads to different ways of thinking or modifying of existing thoughts.
2) Everything you do is wrong, and what you believe is wrong too--basically, they learn from studying and such that families have better chances of healthy children (from all points of view) if two people are around and care for them, society says people have to work, children are (more or less) setup to fail from birth in one way or another. Really--some beliefs are that ADHD and similar may be more environmental (bad or problems in parenting, living situations, etc) than anything. Meaning that parents, either directly (by their actions) or indirectly (through not having a good paying job so children can eat and live in a good place) responsible. Similar are that psychologists know that "birds of a feather flock together" and "opposites attract", people say "that's a contradiction!" yet they themselves have probably used or said similar old wives things without thinking anything of it.
3) The more you learn in psychology, the more you doubt religion, free will, and responsibility. When you discover that a child spanked at the wrong times as a child may turn into a rapist or beat his/her spouse (with high probability), it becomes harder to say that "the majority" of people with "free will" all just "happened" to choose to do the same things. Similar exists with other items--many items with people are known responses, and certain brain conditions lead to certain behaviors that are "against god". Yet if these conditions are biological in nature, and people have no control over it, blaming someone for something they cannot control makes no sense (yet god does) <--- this is one of the big problems within psychology, people who are religious start to have issues and wonder how religion can be right, when they see how many things are outside of a person's control-especially when they can see and test it themselves. Some people actually separate psychology from religion.
And there are other items in psychology that some would hate: babies prefer laying and touching the bare bodies of their parents (i.e. topless or nude), the physical contact allows for better bonding and adaption--this idea (and others in psychology) goes against the "norm" and potentially even religion (ex: at home nudists). Basically psychology learns items that people do not want to know, refuse to listen to, or outright are against because they "know better" even if the proof exists, and they can try it at home.