Strangler fig and rules for killing

From The Trees of Florida, by Gil Nelson, with regard to Ficus aurea, or strangler fig:

The strangler often begins life as an epiphyte….As it grows, it sends down roots which anchor the tree to the ground. Eventually, the expanding roots kill the host and become fused into a bizarrely shaped trunk with massive aerial roots.

We learn about plants and animals. We learn about symbiosis and parasitism.  It's just science, learning the different ways that life forms interact with each other and draw life from each other.  It's just observation and learning.  But when it comes to humans, we have moral judgments. Sometimes, when we kill another living being, it's consider wrong.  There are rules about it though.  We are allowed to kill humans if we call it war or if we call it capital punishment.  We are allowed to kill non-human mammals if we call them things like beef, pork, etc.

When plants or non-human animals kill, it's just how they live, a fact of science.  When humans kill, it's considered evil unless it fits the rules.