How does one fulfil the mitzvah of honoring ones parents who negatively influence your children? How do you speak to them when they are constantly being cruel, deceptive, braggats, and unkind?

Mitzvot, we have to fulfill even when it is not a pleasant experience. Honoring parents is a Mitzvah from Torah and therefore no different to any other Mitzvah in that respect; we have to honor our parents not because they are nice people but because the Torah says so. (YD 240, 3) Nonetheless the mitzvah of honoring parents does not mean you should let them badly influence your children, if you can't help that happening – you can keep your children away from them to the minimum possible. On the other hand, it does mean you should respectfully speak to them, despite their behavioral imperfections or bad character traits. They are your parents; they love you just like you love your own kids,and have devoted a major part of their life for your welfare. Focus on the good side of their personality and try to ignore the bad. This can be an important lesson to your children in how to respect there parents when they grow up and don't always agree with them… (See more in Shulchan Aruch YD 240)