Uncle Dugu did have a motive to kill. In his dream, he killed his wife and Liu Youqiu. He suspected that his wife Hong Qing was having an affair with Liu Youquan, so he killed them. Such a motive for killing is also justified. The key is that Uncle Dugu's memory is chaotic, and some people can't tell whether it is a dream or reality.
The old neighbor heard Uncle Dugu sharpening his knife at night and said that Uncle Dugu had killed his daughter-in-law. Uncle Dugu said that he sharpened his knife and cut meat, and made wonton for his wife. Although there were many knife wounds on the neck of the reddish body, the cause of death was not a knife wound, but a poisonous needle.
That night, Lu Lingfeng came to Dugu's house and found out that he met a masked man in black, who was injured by his poisonous needle, but the fatal wound of his reddish neck was exactly the same.
So it wasn't Uncle Lonely who really killed Pale Red, but the man in black, and we can't completely draw a conclusion. I just don't know if the man in black is killing for others, or if he wants to kill himself, or if the person who really kills is the same as him.
As for Liu Youqiu, according to Jixiang, he found Liu Youqiu's body at the beginning of Youshi, but according to the neighbor's old man, Uncle Du Ya came back in Youshi.
Uncle Dugu doesn't know if this is a dream or a reality. Is there something wrong? When he saw Hong Qing and Liu Youqiu together in the Confucian Temple, he threw the book to them and went home. If so, Liu Youqiu should have died between the time Uncle Lonely threw the book and his return home, so Uncle Lonely didn't seem to commit a crime.