As for a thug like Nijjar: I have no idea how he swung it. It could be any of: mere oversight/incompetence amongst the civil service; small corruption in same; infiltration by Khalistanis of the same; or sympathetic elected officials. Remember that Canada processes 1000-2000 immigrants per DAY, and the idea that we are vetting every single one thoroughly is just silly.
TBH - of all of those, intervention by elected officials seems least likely. Our MPs are not especially hands-on. But who knows? Maybe some idiot really did write a letter of support on parliamentary letterhead in support of a gangster. I expect we'll find out. This scandal isn't going away over night.
Quid pro quo. Same way how USA hosts Gulen. A few greens (or in Canada's case, likely a number of votes) gets passed around under the table and the guy gets "recommended" for a visa. And letters of support for citizenship are often passed for favours. I was offered one by an MP in the UK, and it's perfectly legal. Granted, I'm also not committing illegal shit like blowing up planes over the Atlantic or sth.
So now that the shoe is on the other foot, they just want to treat the Canadians the same way.
From the Indian perspective, Trudeau is basically in bed with the Khalistanis and relies on them for votes and money. Nothing explains people like Nijjar, a guy with minimal life skills, getting a citizenship after having his application rejected twice on grounds for fraud. Just this week there have been reports on a Canadian citizen, from Canada, claiming responsibility for the killing of a local legislator in Punjab for personal vendetta.
And now we are hearing Modi tried to reach out to some separatist factions back in 2015 but his efforts were torpedoed by the Trudeau government who were under pressure from a rival Khalistani faction.
Maybe that's why the Government of India is just saying FU to Canada.