A West Australian man is behind bars tonight accused of murdering a 69-year-old man he knew in Perth's north.It's unclear what sparked the alleged assault inside a state housing property in Padbury.Officers responded to reports of an altercation just after 6.20pm (8.20pm AEST), finding the 69-year-old with life-threatening injuries.READ MORE: Heartbroken daughter demands justice for mum after alleged murderHe was given first aid and rushed to R…