Adding Google Ad scripts in someone else’s website? [closed]

Recently I got to know about Google Ads and how to earn money by putting Google ads on a website.

I have lots of clients and I have access to their sites. What if I put Google Ads scripts on their websites and earn money from the traffic they got on their websites?

For example, I put a Google Ad script into 10 websites, and after one month, I get some revenue from their sites by traffic without letting them knowing that there is Google Ads in their site.

Might any change if they get to know there is Google Ads script in their site? Maximum chances will be that they remove that script. Is there any chances of serious problems or not?