The Sergeant-at-Arms

When the dust settles it appears there will have been just one shooter who killed the soldier and then died in the Canadian Parliament buildings (note that the last account of hijacking a car at gunpoint is out of line with the other witness accounts and makes little sense since you could easily run to the Parliament buildings from the War memorial - the plates were probably stuck over the stolen car's plates so it would not be easily identified as stolen, and removing them may indicate this wasn't intended to be a suicide mission):

"Liberal Senate staffer Stuart Barnable told Metro he saw the whole shooting happen from his office on the East Block of parliament.

“The guy pulled up outside the war memorial, jumped out of his car, got out with a rifle and shot at the guys who guard the War Memorial. He hit one by the look of it. The other (soldier) booted it.”

“He ran back into his car, backed up Wellington and then swerved around and came up and jumped out and ran up towards the House of Commons. He had a long rifle,” said Barnable.

“He was running. He ran back into his car and then he ran up the Hill”

Barnable said the man pulled the plates off a grey Toyota Corolla.

“He ran up the Hill and had a scarf covering his face. I ran down and told security to lock the doors.”

“Everybody is on lockdown.”

Tony Zobl, 35, said he saw the soldier being shot from his office window above the War Memorial.

“I looked out the window and saw a shooter, a man dressed all in black with a kerchief over his nose and mouth and something over his head as well, holding a rifle and shooting an honour guard in front of the Cenotaph point blank, twice,” Zobl said.

“The honour guard dropped to the ground and the shooter kind of raised his arms in triumph.”

Scott Walsh was working in a manhole near the East Block building when he says he heard two gunshots. He then saw an ensuing panic and a suspect he said had long black hair and a face covered with a white scarf.

“He had a double-barrelled shotgun, he was about five feet from me, and he ran right beside us, ran past the woman with the stroller and child.”

The gunman then hijacked a car at gunpoint and drove it toward the Peace Tower."

I think it is amazing that it was the sergeant-at-arms - a half ceremonial and half administrative position whose origins were in actually preventing violence against English members of  parliament - who shot the assailant.