Frankly put, he was a manipulator that started us all down such a traumatic path that one can only begin to heal from it once removed from that plane entirely.
Willem sent Caryll out to the coastal town to ensure his tip from his adventurers-for-hire was true. At the same time, he was having Gehrman train Maria and myself some 'self-defense,' which would become the fundamentals of Hunting.
Caryll sent word back and Willem shipped us out, only able to spare a single horse. The travel by foot took a week, and Gehrman had us going through our training as we made pilgrimmage. The travel took a week, which means poor Caryll was there for three by the time we had arrived. She developed a rather chronic shakiness to her once calmer demeanour, her face sunken and scarred by grief, by newfound Insight.
Soon thereafter we descended onto the beach to see the foretold God that had washed upon the shore, and we were faced with the rampaging offspring it left in its wake. It took the three of us an hour or so to finally kill it in a way that could survive retrieval and dissection, as Willem had ordered us.
We didn't sleep much on the way back. We didn't train. Halfway back we had to start dragging the corpse behind the horse so that we could seat Caryll atop it, for her weakness from barely eating and near constant migraines had begun to diminish her physical strength. I suspect this is when she began to commune with the Cosmos and see runes as a result, but that I can only speculate on.
We returned and the rest of our small research group was waiting, those being Micolash, Laurence and, of course, the Provost. Not a moment was spared for our sakes by he, for his eyes were solely fixed upon his new prize, as was Micolash, though less maniacally as his senior. Laurence was distraught, and only the incessance of Willem was enough to push back his concerns until after the deed was done.
From there the story is a bit more well-known. Willem dissected the Orphan on his very own desk, and we all made ourselves present, save for Caryll who of course needed any mote of sleep and rest that she was afforded by the Old Ones plaguing her so. The discovery of eyes upon the mind was enough to send the three of us back to the village, and that is a time I shall not revisit here.
So, with all that said, what do I feel about Willem? These are my first thoughts of him when he is mentioned, how he put me and those I cared about in not only physical danger, but directly interweaving us into the spiral that Yharnam as a whole sunk into. There was a time where I respected Willem, he who housed me when no one else would, who turned my life around for the better and respected my intelligence.
Then he betrayed my trust. He is responsible for Maria's death. So take your best guess as to how I feel about him and multiply that by how you feel about the person who's wronged you the worst, and to be safe, triple that.