Haunting last text of American sniper Chris Kyle before he was shot dead

An American hero was murdered in cold blood while attempting to help the man who killed him.

You've probably heard of US Navy SEAL Chris Kyle, and if his name doesn't ring a bell, you'll probably be familiar with the Oscar-winning movie American Sniper directed by Clint Eastwood? Yes, the one starring Bradley Cooper - who plays Kyle.

The former US veteran claimed to have racked up 255 kills in combat, although the US Department of Defense has only officially been able to confirm 160 of them.

Having resigned from his duties in the field, he began running sponsored outings where he would meet veterans and take them hunting or to shooting ranges.

It was due to the 38-year-old's empathy that he met the man who would go on to murder him - Eddie Ray Routh.

He was an ex-marine who was in and out of hospital suffering with PTSD - due to him serving in Iraq and also helping in the aftermath of the Haiti earthquake.

His mom approached Kyle asking him if he could help her son cope with his trauma - Kyle was familiar with Routh's mom as his children went to the school where she worked, and so he did her a favor.

American hero Chris Kyle was murdered in cold blood. (Chris Haston/NBCU Photo Bank/NBCUniversal via Getty Images via Getty Images)

Kyle, alongside his 35-year-old friend Chad Littlefield, took Routh to a shooting range at Rough Creek Lodge and Resort, Texas, when tragedy struck.

He was behind the wheel with Littlefield driving shotgun and Routh behind them - but there was a sense of nervousness in the air.

So much so that Kyle text Littlefield while driving, in what would be his haunting last text.

It read: "This dude is straight up nuts."

To which his buddy replied: "He's right behind me, watch my six."

For those that aren't familiar with the phrase it is military slang asking for someone to watch their back.

Kyle pretends to aim down a barrel of a gun, pictured with Dean Cain. (Trae Patton/NBCU Photo Bank/NBCUniversal via Getty Images via Getty Images)

Shortly after the texts were exchanged, when they reached their destination, Routh fired seven shots into the back, shoulder, head and hands of Littlefield, before opening fire on Kyle unloading six bullets to his head, chest, shoulder and right arm.

When cops caught up with Routh they placed him in handcuffs and it was then that he complained to the police officers that the two men he killed 'wouldn't talk to me'.

He later revealed to a forensic psychologist that he believed Kyle and Littlefield were planning on killing him due to the number of guns they were bringing with them.

At his trial Routh pleaded not guilty by way of insanity, but a jury found him guilty and he was sentenced to life imprisonment with no opportunity for parole.