Why 3 Bullets rather than another method

Why 3 Bullets rather than another method I've wondered why the person who murdered Karmein chose to kill her by 3 gun bullets rather than another method such as strangling or suffocation etc?

If it was attributed to the Cruel offender, in the past, despite his unspeakable assaults, it was reported he never appeared to be rough and vicious with the previous girls.

Interestingly, with the other invasions, the offender always had a gun yet when the offender apprehended Karmein and her sisters, he only had a knife yet it was a gun used in her murder. When he took her I read he was quite rough and didn't take a change of clothes. He may have known even then she would not need them as she was not returning. Therefore whether or not she would later pull his balaclava off was not the reason for her murder.
Excluding other things, Does the gun absence at Templestow and aggressiveness possibky point to a separate offender?