Are you saying a "Canadian Terrorist" murdered Nijjar? The article you link says nothing about any country (except India, Bangladesh and Sri Lanka) disputing Canada's claim.
This book has more details about the movement: https://www.amazon.in/Blood-Fifty-Global-Khalistan-Project/d...
It is a significant movement in Canada. https://en.wikipedia.org/wiki/Air_India_Flight_182
Also, the veracity of a claim does not depend on who is making it or who is disputing it. The accusing investigation agency has do a proper investigation and submit proofs and ask for extradition.
Several countries examined Canada's evidence and found it satisfactory. Your government conducted an extra judicial execution of a Canadian citizen on Canadian territory. You are the baddies, even if your government assures you of the opposite.