It means that, besides being less deadly to humans than the Japanese giant hornet, all of the other hornets follow it at a very long distance on the deadliness list. 'By far' means something like 'very much', or 'to a great degree'.

It means that the Japanese giant hornet kills many more in comparison. For example, one kind of insect kills 10, another kind of insect kills 20, and the Japanese giant hornet kills 1000.

"by far" used for saying that something has much more of a particular quality than anything else.