The first day of the Launceston Supreme Court trial heard evidence about the Kings Meadows man's mental state and behaviour in the months and hours leading up to the alleged incident.

Mr Barker, 31, has pleaded not guilty to murdering Dale James Watson and assaulting Timothy John Bumford on March 9 last year.

The court heard Mr Watson was walking on Knox Street at Prospect Vale when Mr Barker revved his engine, mounted the kerb and hit the pedestrian.

Crown prosecutor Linda Mason told the jury the force caused Mr Watson to hit the bonnet and be catapulted onto the road.

Shortly after, Mr Barker allegedly assaulted Fastway courier driver Mr Bumford near Sacred Heart Primary School on York Street in Launceston.

While giving evidence on Monday, Daniel Yould told the court Mr Barker had been on edge since he was bashed by four men in 2009.

He was diagnosed with post-traumatic stress disorder after the assault.

Mr Yould said he told Mr Barker he was concerned about him in the days before the alleged incident.

Mr Yould said he received two text messages from Mr Barker in the early hours of March 8.

One text message didn't make sense, but the other asked Mr Yould to visit Mr Baker that day, the court was told.

Mr Yould said he went over after work about 8pm and recorded his conversation with Mr Barker in the hope it would help his friend get the assistance he needed.

Mr Barker allegedly did most of the talking, with Mr Yould saying he was confused by a lot of what was said.

The court heard Mr Barker said he studied all the world's religions, was obsessed with numbers and thought his house had been bugged.

Defence lawyer Evan Hughes told the court Mr Barker's mental illness was echoing in his mind at the time, before cascading into a loss of understanding.

The court heard the Barker family and their son's friends had planned to hold an intervention about his mental health at 3pm on March 9 but never got the chance.

The alleged hit and run happened in very close proximity to where Mr Barker was assaulted, the court was told.

Mr Barker's parents Peter and Monica supplied statutory declarations to prosecution ahead of the trial and they were both read to the court on Monday.

The father detailed several events which heightened his son's mental health issues in the lead up to March 9, including his beloved cat being run over, his aunt having health issues, a friend's younger brother dying and a meltdown at work.

Monica said her son's mental state had become more and more unstable, he started using terrible profanities and was living in fear he would be attacked again.

Mr Barker's mother described him as happy-go-lucky before the 2009 assault and withdrawn afterwards.

The trial before Justice Robert Pearce is expected to go for 10 days.