Unlike Snowball in the first movie, Sergei is not redeemed by the end of the movie. This is because Sergei is Snowball's foil. Sergei is a less redeemable villain because he is a human and just so happens to embody that some humans do have a tendency for cruelty. Snowball, on the other hand, is an animal abandoned by his owner, and abused animals act "cruel" only because it's based on how they are treated (Snowball was embittered by his past abandonment until he found his Forever Home with Molly).
Sergei's ominous dress style is reminiscent of an evil witch or creepy magician. Snowball's original, neglectful owner was a stage magician, so that's the rabbit has crossed paths with creepy entertainers in capes twice.
Hu the tiger finding a home in the cat lady's house may seem like a happy ending. However, even considering that Hu is friendly and non-aggressive for a tiger, it still raises questions about what could happen if the neighbors or local authorities find out, especially as it is illegal in many countries to keep wild animals as pets, particularly large and potentially dangerous carnivores (even if Hu is the exception to the rule).
If the local authorities found out, Hu would most likely be humanely transferred to a zoo/animal sanctuary where he would be properly cared for. Unfortunately, the cat lady herself would get in trouble for having him, and her housecats might well end up in animal shelters if she's exceeded the limit on the number permitted by law and/or her lease.
Sergei threatens to turn one of the wolves into a coat for his monkey, Little Sergei. Sergei's coat is black. The wolves are black. Is Sergei's own coat made of a wolf, or wolves, that displeased him in the past?
