Valknut is not a Viking Age word. It is a modern term, and no record survives of what the symbol was called when it was in use. This is worth dwelling on, because the name is often glossed as knot of the slain and that gloss is then treated as evidence of meaning. It is not evidence of anything; it is a modern coinage being read back into the material as though it were a source.
The association with Odin is an inference, and a reasonable one. The figure keeps company with imagery of hanging, sacrifice and the dead, and Odin is the god of exactly those things. Hilda Ellis Davidson proposed a link to Odin's stated power to bind and loose the minds of warriors in battle. Rudolf Simek suggested more cautiously that it relates to practices surrounding death. A comparison is sometimes drawn with the Prose Edda's description of the heart of the giant Hrungnir as three-pointed, which is a genuine textual detail but is not said to be this figure. These are scholarly proposals. They are not attested doctrine and should not be repeated as such.
The modern difficulty is separate and should be stated separately. The symbol has been taken up by some white supremacist groups, and it is catalogued as such by the Anti-Defamation League and comparable bodies. That does not make it a hate symbol in the way that certain other appropriated signs have become one: it remains in ordinary use by heathen and Asatru practitioners, by museums, and as a Scandinavian decorative motif, and the organisations that list it say so explicitly. The appropriation is a fact about the present. It is not a fact about the eighth century, and the two should not be allowed to contaminate one another in either direction.