why people speak ill of others?

I live with my in laws to help them out due to my father in law being disabled and unable to do much. I help out around the house a lot and pretty much home a lot of the time to watch over him and clean up and what not. my father in law and I started off with a good relationship but slowly over time I had lost a lot of respect for him. he speaks ill of everyone in our family and worse I have heard him speaking to my husbands aunt about me. saying nasty things. I just can't understand why he has such grudges on people. his sister I heard has a problem with me now too and I don't even know her. she lives in a different state and calls her brother twice a year. so I'm wondering what it is that gives others the right to judge me or say things about me that aren't even close to the truth? I still speak to my father in law and help him out as much as possible but due to his negativity have decided that I had to distance myself for my own sanity. I'm a bit sensitive and I take things personally even when I know I haven't done anything wrong. I guess I just want a little insight on what everyone thinks are some reasons for bad mouthing others? especially others that are trying to help you.