Even if you had good intentions in using a modified client, the rules clearly state that you shouldn't be using one. If you find a player to have a modified client you don't need to use a client to prove they have one as well, you can take a short clip of them using the said client, or notify staff via /report. Excessively reacting to the player is exactly the reply they're looking for, it's always best to stay calm and collected in situations like these.
Do you understand the reasons that I've listed and agree to comply with them?