ANSWER - Your uncle is both wrong and right. He's incorrect to imply that all witches use animal sacrifice (they certainly don't) but he is correct in believing that some witches do use sacrifice as a part of their path. However as with most things in life - generalising isn't very helpful. Individuals adapt their practises and beliefs to what suits them and the fact that some witches would do the things your uncle describes doesn't mean either that others have to do it or that it is a necessary part of what it is to be a witch. I don't perform animal sacrifice myself for example - I'm the kind of witch who'd spend 20 minutes helping a spider out of the bath so it doesn't drown. The thought of killing a creature turns my stomach. I do utilise dead things in my magic but they are without exception always dead when I find them.
The belief that witches harm none is intrinsically a Wiccan belief. Wicca is the modern practise of the craft and far more concerned with moral law than Traditional Witchcraft which is a secular magic path focusing on the belief of the individual rather than a shared group belief. Wiccan witches follow the Wiccan rede, believe in the sanctity of life, the principle of harm none and the idea of the threefold law of return. A Wiccan would not use an athame under any circumstances to kill an animal and most Wiccans dislike the idea of their athame coming into contact with any blood, including their own. It sounds to me that your own beliefs fit this persuasion quite well and it might be worth getting your hands on a good quality rudimentary book on Wicca (normally anything by Scott Cunningham is well thought of) and to perhaps encourage your uncle to have a read as well. If he is mistaking the Wiccan belief system for the less morally structured forms of witchcraft that may stray into areas you/he would find uncomfortable then it is understandable that he has reservations. From my knowledge of the Wiccan religion (which I do not personally practise by the way) I doubt very much he will find anything that would cause him concern if he were to read more about it. Wiccan witches are very focused on doing good in the world and not using their magic to harm or to hurt. A Wiccan for example is very unlikely to use hex/curse spells and would consider animal sacrifice to be a total violation of their harm none law.
You ask if there are witches who don't believe in harm none - well yes, there are. As I say, harm none is primarily a Wiccan belief and most witches practising crafts from older times do not make the differentiation between light and dark magic that the Wiccans do. Traditional witches view magic as a neutral force to be shaped with the will of the witch. So magic can be used for harming and for healing for example but it is not the magic itself that is good or bad, it is the intent of the witch. A Wiccan witch (and there are substantially more Wiccan witches in the Western world than there are any other type of witch) would use that same force primarily to bring about positive results that they deem to be within their sphere of harm none. That is not to say that Traditional witches don't observe their own moral creed, morality for some is still a concern but the beliefs are more centered on what the individual witch deems appropriate rather than a law of which all would abide by.
I'm glad you asked the question because in your case I think some reading on Wicca will help you to define the path you wish to go on and I think your uncle will be reassured that you are not interested in practising the voodoo/shamanism types of magic that Jamaican witchcraft would fall into. Keep the communication lines between you and your uncle open, be open and upfront about your discoveries as you progress on your path and let him share in your journey of education and exploration. The media have done a lot of work in portraying witches as sacrificers of chickens and the like and it is more than understandable that those not practising the craft have a negative view about what it is witches actually do. Your uncle sounds like a man who finds it all quite interesting and I think if you are willing to be open and share from your end you will find that he becomes more accepting of who you are and more tolerant about the path you wish to pursue.
And one final point - please never worry that you have to do anything other witches do. You can be a valid witch in your own right without ever having to do anything that you don't want to do or that feels wrong to you. Listen to your intuition -if something feels wrong, don't do it.