Most people would be surprised how advanced mammals in particular are. Dogs and cats (the most common pet animals) have extremely highly developed sentience, equivalent to that of a small child, and I don't know anyone who thinks it's ok to physically or mentally abuse or mistreat a small child. They deserve compassion and respect just as humans do.
Personally, I'm angered even more by abuse to animals than to (adult) humans for the simple reason that humans can understand why something is happening. They have the intellectual capacity to reason and the ability to communicate their problems and fears via language, whereas animals (and small children to a degree) do not.
I will also admit to judging humans largely on their treatment and empathy towards animals. Anyone who lacks compassion for them, I cannot respect, even if they share my views on everything else. Likewise, those who are completely the opposite of me in terms of how they think or believe politically/religiously/whatever, I can still hold them in massively high regard if they have empathy and compassion towards animals.