Because communism is inherently a violent and paranoid ideology. It's blatantly obvious even in the foundational literature from Marx. Marx openly advocated violence and threatened violence against anyone who was opposed to the proletariat.

Not only that, but he construed a framework in which anything opposing the proletariat, including "class antagonisms" that could be literally anything, should be opposed with force or the threat of force. Things like religion, private property, or anything that is not for the revolution is necessarily considered against the revolution.

When the foundation of a worldview is steeped in violence and using violence to get your political way, it's no surprise that the implementation of that worldview results in bloodshed and power vacuums.