Reverse Engineering Illegal?

why would you bother doin this

first check if the api is public and that its not a sh*tsploit that uses the public jjsploit api if its not public then ask one of the devs but if they didnt make it public you probably cant use it, dont know if its illegal or not tho..

not worth it you will prob end up with - 9999999999999 rep if you do and become the next wonder

That's pretty likely. 
I assume you already have reverse engineering experience (considering your question), so the better approach would be creating your own dll instead, and using open source dlls as reference if you're having issues with understanding what to do. You can also try asking experienced devs as well, just don't beg for spoonfeeds. 

It's not illegal unless you've agreed to terms explicitly stating that you will not reverse engineer said program, ex: GTA V states that you may not reverse engineer their program when you first open it, and somewhere in the rockstar TOS when you're making your account it says the same.

Some people will argue copyright but I doubt anyone has filed copyright on their software in this community and even if they did it wouldn't matter because you aren't actually using their source code, you're recreating something they made which is totally legal as long as you are not breaking any copyright laws