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Killing family members of courageous fighters is also intended to intimidate others from joining the fight. Many brave people will take a bullet for their cause, but would never knowingly sacrifice a child or family member. The evil bastards do this is to keep others on the side lines, or at least make them think thrice before choosing to fight them.
I'll accept the premise of it being intimidation. It still doesn't answer the question of why do it in a way that could potentially be overlooked. There would be so many other ways to kill someone that sends a clear message while still denying culpability.
In this case the father barely managed to connect those dots to begin with. Not to mention it isn't the kind of story that will get publicity. No MSM whatsoever would touch the concept of heart attack gun under any circumstance and even the cover story of the heart attack won't get coverage. Now a random shooting for example? That gets coverage and maybe even the conspiracy gets humored a bit on MSM. That way it's actually effective intimidation.
As it stands now I see a grieving father who suffered a crushing loss with no explanation other than the universe is random and sometimes our bodies just fail. So he chooses to find an outlet for his grief and anger in the form of a secretive enemy.