For what? Using his constitutional authority? You need to impeach and remove somebody for a reason besides disagreeing with somebody politically.
For using his constitutional authority unwisely. Pardoning racist thug Arpaio is one more bit of proof (not that more was needed) that Trump is unfit to hold the office. That alone is sufficient justification to impeach him. That he has not been impeached should to destroy the Republican Party in coming years.
But that's not illegal, it's still legal to pardon Arpaio whether or not you like the man. You can't be impeached for pardoning somebody that you don't like. It's not a crime.
You don't seem to understand how impeachment works. Here's the relevant passage:
As the relevant wikipedia page makes clear, High Crimes and Misdemeanors covers all or any crime that abuses office.
https://en.wikipedia.org/wiki/High_crimes_and_misdemeanorsDeciding whether pardoning a racist criminal thug and campaign supporter who committed crimes under the cover of law warrants impeachment is very much up to Congress. But they certainly
can do so, and if they do they're just as much acting under their authority as Trumpolini is by issuing this pardon.