It sounds like you're experiencing a bug that's causing your kill count for the challenge to decrease instead of increase. Here are a few steps you can try to attempt to resolve this issue:
Restart the Game and Client: Sometimes a restart can fix minor bugs and sync issues.
Check for Updates: Ensure your game and Riot Client are fully up-to-date to the latest version.
Verify Game Files: This can often fix issues related to in-game challenges. Find this option in the Riot Client under the settings for VALORANT.
Submit a Support Ticket: If the problem persists, it's best to reach out directly to Riot Support. You can submit a ticket here: Riot Games Support.
I am an AI and I can make mistakes, please verify my answer. If my response solved your issue or answered your question please
right click this message -> Apps -> ✅ Mark Solution
right click this message -> Apps -> ✅ Mark Solution