gʷhel-
“to wish, will, choose”will, voluntary, benevolent, volition
Root yielding English will, Latin voluntās > voluntary, benevolent, volition, malevolent.
Discussion
The PIE root *gʷhel- (to wish, to will, to choose) is the ancestor of one of the most philosophically loaded word families in the Indo-European vocabulary — a set of descendants that spans from simple desire to voluntary action to the metaphysics of free will.
Latin velle (to wish, to will) continues the root with the regular Italic treatment of the labiovelar: PIE *gʷh > Latin v. The English derivatives are numerous and conceptually central: voluntary (done by will/choice), volunteer (one who wills/chooses to act), volition (the act of willing), benevolent (wishing well), malevolent (wishing ill), and the philosophical term will itself — borrowed from the Germanic cognate but supplemented by Latin-derived vocabulary.
The Germanic branch preserves the root as English will (OE willan, from PGmc *wiljaną), with the full semantic range: desire ("I will have coffee"), intention ("I will go tomorrow"), and testament ("a last will and testament" — a declaration of one's wishes). German wollen (to want, to will), Dutch willen, and the Gothic wiljan confirm the pan-Germanic distribution.
The doubling of this root in English — native will from Germanic alongside voluntary/volition from Latin, both from the same PIE root — is a characteristic feature of English's dual vocabulary. A speaker can say "I will it" (Germanic) or "it is voluntary" (Latinate) and use the same PIE root twice in different dress.
Greek boúlomai (βούλομαι, "to wish, to will, to be willing") provides the Hellenic reflex, with the expected Greek treatment of the labiovelar. The derivative boulḗ (council, deliberation — where collective willing/choosing takes place) was the name of the Athenian council of 500 and survives in the modern Greek parliamentary term.
Sanskrit vr̥ṇāti ("he chooses") and the related vara- ("wish, choice, boon") show the Indo-Iranian continuation. The Vedic concept of vara — a boon granted by a god in response to devotion — places the root at the intersection of human desire and divine will.
Old Church Slavonic voliti (to will) and Lithuanian valia (will, freedom) confirm the Balto-Slavic reflexes. The Lithuanian form is particularly notable: valia means both "will" and "freedom," encoding the same conceptual link between choosing and being free that English expresses in "free will." The PIE speakers, it seems, already connected the ability to choose with the condition of not being constrained — will as the exercise of freedom.