Do not speak ill of the dead

I get this but why do some people lie to speak favorable of the deceased?

A co-worker died about a year ago. He was a very active 50 year old guy and had a massive heart attack and died suddenly. I'm the first to say it was sad that he passed away and that is the only comment I have made at work about it. But honestly the co-worker was the biggest A-hole I have ever met in my life. He was a trouble maker, passive aggressive, shite stirring instigator. He was warned several times and was finally fired. I never figured out what he did as the last straw but I knew he was lucky to last as long as he did. On his last day of work here they brought in some catered food and even a good bye cake since he had worked here for 10 years. Well, the co-worker refused to go to his farewell party. He sat in the other break room eating his TV dinner alone. And the at the end of the day, when there was just a few people left, he goes and takes his half eaten cake as he walks out the door. That Friday they threw a happy hour celebrating him being fired. The co-worker died 7 months later. I did not go to the hh but it's funny hearing people that went to the hh saying the absolute nicest things about this guy one year after he passed. I just smile and shake my head when I hear these comments especially from certain people.