Butter from animals, like all animal products, increases the risk of heart attack:
and causes other health problems: http://www.adelicatebalance.com.au/
Most importantly, it is not right to use, exploit, enslave, hurt and kill animals for butter, which is done in the milk industry. Cows are forcefully impregnated, their reproductive organs are exploited and their calves are stolen.
Cows, horses, and other animals are consciousness just like we are, and they have the same right as us to live their life without being exploited, enslaved, hurt and killed. They value their lives, family and friends, and we have no right to take this from them.
I recommend this lecture: http://vimeo.com/17538152 and www.vegankit.com
Have a nice day!