what's the etymology of 'illocutionary'? Since 'allocute' is basically 'to speak', does the 'il-' prefix form the negative connotation?

nope. it's the assimilated form in L. of the prefix 'in-', meaning in/on/into/upon; so an illocution is (in philosophical terms) an act such as ordering, warning, undertaking, performed in saying something.

"To perform a locutionary act is in general... also and eo ipso to perform an illocutionary act, as I propose to call it." -J.L. Austin, How to do Things with Words

(not to be confused with 'perlocution', an act such as persuading or convincing, that may or may not be successfully achieved by an illocutionary act such as entreating or arguing.)