unnecessarily

needlessly, causelessly, without occasion, by chance, carelessly, fortuitously, casually, haphazardly, wantonly, accidentally, unessentially, redundantly, inexpediently, uselessly, exorbitantly, superfluously, undesirably, objectionably, disadvantageously, optionally, avoidably, without cause, without reason, gratuitously; see also foolishly.

Antonyms necessarily*, indispensably, unavoidably.