One of the main sources of bad kamma in Buddhism is doing wrong to your parents and not taking care of them. Buddha said it is very very hard to repay your parents, however how should one behave if he or she has bad parents? Maybe a parent that was an alcoholic or didn't take care of him properly, abandoned him, banned from his house or maybe stole from him? There are countless possibilities, what to do?
Even if your parents abandoned you the day you were born, you still owe them. The only way to repay them fully is to make them understand the Dhamma. If your parents are alcoholic, take them for rehab. If they steal money, don't keep money where they can find. Likewise, address each issue accordingly. Advice them with the Dhamma and show them the right path without getting angry.
There is a aspect of Buddhism which actually views difficult people (including parents) as being helpful and we should be grateful to them. Forbearance (kṣānti) is one of the six perfections. To illustrate let me quote from the Bodhicaryāvatāra, Perfection of Forebearance (verse 107)
So we should be grateful to difficult people even if they are, well, difficult.
But realistically we aren't all able to do this, certainly not in the initial stages of practice. Also difficult relationships with parents particularly colour our lives and are difficult to get past. When I have had difficult relationships (not with parents admittedly) then sometimes the only thing to do is take yourself out of that situation until such time as you are better able to cope positively with it - and if that time never comes well at last you are out of that situation and maybe better able to practice positively generally.
Also Buddhist teachers like Jack Kornfield actively recommend counselling to go alongside Buddhist practice and the two work side by side. This article illustrates Jack Kornfield's position which I'll quote a brief section
However bear in mind that....
In my opinion, when things come up in meditation practice, why not use every facility that is available to you to come to terms with it? I appreciate might be a controversial view. People are obviously welcome to disagree with.