In the first stanza, Hughes suggests the violence possessed by pikes:
"Killers from the egg: the malevolent aged grin.
They dance on the surface among the flies."
The word "killers" needs no further explanation. The pike is born with an instinctive desire to kill and "malevolent" suggests that the fish has an inherent evil; it is naturally vicious.
Stanza four further develops the idea of how malignant the pike can be:
"The jaws’ hooked clamp and fangs"
It is clear that the pike's jaws are specially designed: it first "hooks" and then "clamps" down, finally finishing off its prey with its fangs. This tells us that the pike can effectively capture and destroy whatever prey comes within its reach. There is no, or very little chance of, escape. The image of prey trapped in those fearsome jaws is both violent and horrific.
In stanza six, Hughes emphasizes how ruthless pikes are:
"And indeed they spare nobody."
The pikes do not discriminate. They attack and eat whatever they can, cannibalizing even their own kind, as is made clear in stanzas five and seven:
"Three we kept behind glass,
... Suddenly there were two. Finally one" (stanza 5)
"One jammed past its gills down the other’s gullet" (stanza 7)
It is clear throughout the poem that pikes are vicious and terrifying creatures and that they are naturally inclined to be so. These fish are a metaphor for the instinctual violence that exists in nature which is, no matter how frightening, essential for survival.