Looks like he wanted to get revenge.
2 seconds sound clip from Scooby-Doo, Where Are You! - Bedlam in the Big Top (S1E10).
You can hear this line at 00:19:51.522 in the Blu-ray version of the episode.
Quote context
[...]
- Look what Shaggy and Scooby brought in on a leash.
- They got him! But what's going on?
- Simple! He hypnotized himself!
- I know that man. He's Harry the Hypnotist.
- He had an act with us, but we caught him stealing and sent him to prison.
- Looks like he wanted to get revenge.
- As a ghost clown, he knew he could ruin your circus.
- Well, it was great work, kids. Thanks to you, my circus will open this year after all.
- Shaggy and Scooby sure were mysterious about this big surprise they have for us.
[...]