I'm going to fire one of my employees who has a bad track record
Is it because of their performance?
Yes, I need someone who is more active every day and this person always has a bad attitude.
That is a reasonable reason to fire someone, assuming you've already spoken to them about their performance. If they have a bad attitude and have continued to have a poor record, then you shouldn't be obligated to support them.
Their lack of performance and grumpy mood is their own fault.
I'm sorry this is happening at work. If you spoken to the employee already, you need to take additional steps to make sure the work is being done. You should only try to employ those that are going to make your company better.