All I can say is... HOLY F-ing SHIT

I think, initially it was to defeat Inaho. Keep in mind that Troyard hadn't yet adopted Slaine when he shot off the bullets, meaning that if Troyard had died in combat, Slaine would have gone back to being a nobody. Once he was adopted, he adapted the plan to try and kill both Troyard and Inaho.

Also, Inaho told Inko that if she hadn't been there, he would have lost. So basically Inaho got lucky.