Bad toddler behavior help?

Hello everyone, I have a very big concern and I need help. It's about my son. We have very big problems with him. My girlfriend had a partner at the time who treated her very badly and the little one always saw this – not everything, but he saw a few handshakes, that's what I'll call it, and how often his mother cried.
In any case, when he was 3 years old, the little one started hitting his mum and his older sister, who was 6 years old. He threw objects at his mum, hit her in the mouth with a coat hanger, threw a plate at her foot, hit his sister on the head with a bucket and hit her in the face with a pole, leaving a cut under her eye.
Then the little one started shaking a pregnant guinea pig until it had babies…. Then he took a baby (5 weeks old at the time) and threw it down the stairs and shouted, "Mummy, now we don't have any guinea pigs anymore."… She then gave the animals away…. But the little one grew up with dogs and cats. Since birth, he's had a dog, 2 cats and 5 guinea pigs. His grandmother lives next door and has 6 dogs herself. So he knows how to be with animals because he was taught very well, as is his sister, she's the complete opposite. Anyway, the cat was pregnant, he kicked her in the stomach and said, "Mummy, baby, it's finally dead."…
After that he didn't do anything for a long time, or rather didn't do anything to any animal, until yesterday… he strangled our cat and she had to have emergency surgery. He just laughed and said it didn't matter, she was still alive… We are all very shocked.
We've had someone from the youth welfare office for seven months, but she's of no use to us. She comes every two weeks for an hour. Furthermore, we went to the social services three months ago and of course I called the psychologist straight away today. She said we should explain to him normally that we shouldn't do things like that, BUT how often have we had this happen… his sister is already afraid of him (well, he also has outbursts of anger, which I haven't mentioned directly). He just screams as if someone was going to hurt him, but he's only at home or with grandma and grandpa, so with everyone else he's a model child. The others only believe it with video evidence… my wife is toying with the idea of ​​putting him in a foster family, but she can't bring herself to do it, which is understandable… but she's just at the end of her rope and the older one too. She's only been sleeping with grandma and grandpa for a long time and doesn't want to go home anymore. Is there anyone out there who can give us tips on what we can do or why he is the way he is, because he's 4 YEARS OLD…. how can a 4 year old be like that…

To reassure you, she brought all the animals to her grandmother yesterday, and there are no more animals in the house…