I think William falls into the same category as Finral and Lolopechka—too kind to be able to learn offensive spells.
William is always called kind by everyone around him. By Julius, by Yuno, even Patri acknowledges that William is different. Zenon calls him a fool for prioritizing others rather than himself
William berates himself for his kindness (indecision between elves and humans), and Zenon calls him out as an idiot for his kindness too. But William then defends this, because Zenon insults the people who showed kindness toward William.
William can take insults. But he won't let Zenon talk down the others.
If you look closer at William's spells, none of them are offensive. He can heal, he can bind, but they aren't full-on offensive.
When he dealt with Broccos (Ch. 77), he just used his magic according to its nature—trees need nutrients to feed on. Being trees made of magic, mana becomes obvious as a source of energy to them.
It wasn't a direct offensive spell, it was literally him growing a tree and using a person as the source. He does this earlier in Kiten too, using people's magic power in his vicinity to fuel his big tree.
Like Lolopechka, he could make things out of his magic that were intended for offense. She could make knight dolls, but could only make them look like they were attacking. William learned to make a sword out of his magic, but never used it himself until Lucius.
And really, is taking a sword and stabbing someone the same as an offensive magic spell in a grimoire? It was a matter of Creation Magic, and even Lolopechka could technically make soldiers, but couldn't be like Princia.
Whenever William uses his magic to beat an opponent, he just immobilizes them. His roots or vines move toward the target, and he technically wins via binding or draining their magic power afterwards.
Like Lolopechka, his spells look like attacks, but aren't actually that. William has incredible magic control, and his stats show it as a 5 (the highest it can go).
In the defending of Kiten, William did a huge spell—and he left no property damage. He didn't destroy any buildings. Look at the difference between how the spell was received, to the Diamond mages and the Clover citizens.
The only people who were hurt by William's magic were Diamond mages; Clover citizens were completely fine and left alone. For example, Asta, Langris, Yuno, Charmy, and Finral, weren't impeded by the tree at all. It clears out around them.
In the 2 images above, Yuno is left alone despite the tree having grown heavily toward him. The Diamond mages were clearly picked out from where they were mixed with Clover citizens.
The people who destroyed the area were Diamond mages, and people like Yuno and Langris. There's no explicit showing of William himself actually destroying buildings or hurting civilians (I'm going based off the manga, I don't know if this proves the same in the anime).
Since William doesn't have clear offensive spells, and the nature of his magic is more protection (bulky, strong wood), that's probably a reason he put Langris as Vice-Captain (other than him being the vessel for Patri's cousin). Langris has offensive magic. He can deal with what William lacks, which is pure offensive power.
William has great magic control, and he uses that to not hurt people or damage things unnecessarily
His magic only looks like attacks, something Lolopechka also does
When on the topic of William, everyone always points out how kind William is
With the earlier points, I think William is too kind to be able to learn offensive magic
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