In other words, he was already known to be trouble, likely obtained the gun illegally and what he intended was not good.

Via Daily Wire:

The suspect has been identified as a troubled student who was in fact still enrolled at the school but was “working independently out of the principal’s office.” Illinois State Police say he faces three felony counts of aggravated discharge of a firearm and may face further charges. He was shot twice, once in his shoulder.

A woman who identified herself as his mother spoke with reporters Wednesday. She described her son as having been “very sad” for a long time, in part because of feeling ostracized and bullied. WGN-TV reports (name of suspect redacted):

The woman said her son was bullied and ostracized at school, and was beaten up in October. Officials initially described [the suspect] as a former student, but the woman said [he] still attended Dixon High School, where he worked independently out of the principal’s office. He was slated to attend graduation rehearsal Wednesday, the woman said, but instead showed up with a gun.

She told reporters that she does not keep a gun in her house and does not know how he obtained the weapon.