I wonder if he's a Vietnam veteran who deserted and went to Canada, as tens of thousands of men did. The timeline would fit.
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This is a possibility. President Ford offered an amnesty for Vietnam era deserters in 1974 but there were conditions including 2years public service so many in Canada did not take it. In 1977, President Carter gave a very generous amnesty to draft evaders but it did not cover deserters.
In 1977, a Deserter who was not happy in Canada but had not taken advantage of the 1974 offer would have been arrested and turned over to the Military if he tried to return to the US. It is possible that that a deserter living in Canada might set up a false identity in order to return to the US. The consequences of getting caught would not have that severe however: probably less than a year in the Stockade and a General Discharge. It would not have justified multiple murders.
Apparently about 90,000 draft dodgers went to Canada but only 1,000 deserters. I would think the FBI would be able to identify any that disappeared around 1977 when Evans turned up in Manchester.