How did Kirito's Illusion actually kill?
Rather than an illusion magic, it seems what Kirito used is a transformation magic instead. The following is based from the 3rd novel.

&quot;Well, I somehow remember the battle earlier. I used the magic Yui suggested, and became very big. Since my sword disappeared, I had to use my hands...&quot;
&quot;Also with the teeth bite yo~&quot;
On Lyfa's shoulder, Yui added with pleasure.

Remember that Yui is an AI from Aincrad and that upon arriving at the Alfheim world, Kirito noticed that Alfheim is based on Aincrad, including the skill sets and player data.

In the skills window, I saw several skills side by side. These skills included: 1-H Swords, Unarmed Skills, and Parry, the Combat skills, as well as Fishing, a support skill, but the proficiency values were abnormal. More were just about 900, and some had even reached 1,000 and had a tag denoting MASTERY.

Later when asking Yui, Yui confirmed that.

&quot;It seems this world is based on a copy of the Sword Art Online server.&quot;

This explains how Kirito can turn into The Gleam Eyes. Since it is a copy of the SAO, some resource file might be still left intact. While it is not normally accessible by the players, with Yui giving assist, it is possible that she hacked the system a bit and granted access to Kirito to transform into The Gleam Eyes. This wasn't explicitly mentioned in the novel, however.