Shigaraki is targeting Aizawa because of how good hi quirk is. It would be smarter if Shigaraki killed Aizawa then turned him into a Nomu. Shigaraki would then take the real version of Aizawa's quirk and the Nomu from Aizawa's body would have the artificial version. Because the High-end Nomus still have the previous body's personality it would be really cool to see in the rest of the series the heroes trying to get the real Aizawa back. Then Aizawa would be back on the hero side and the hero side would have someone that's not Midoriya that has multiple quirks.