On Matthew 28:19 Jesus already handed over His services to his servants to go, minister, baptize and made all His disciples. So, this has to be fulfilled and Anania got a call for that to minister Saul. Secondly, even if Paul was just converted and went to Christians, he might not be accepted because of his background. So, it had to be witnessed by Servants.
2. Anania just argued from his limited knowledge of Paul's past experience. Judging someone with past experience is very common even today among Christians. But our past is not a guarantee for today!
3. He obeyed Jesus because he trusted and believed in Jesus expecting He took responsibility if things would had been other way round!
4. He minister Paul and nurtured him, for sure!