There are several places in the book that shows that Long John Silver is a ferocious person. But it is important to mention that he is a very nuanced character. For example, he quickly gains the respect of Captain Smollett and Dr. Livesey, and he has a good relationship with Jim. In fact, it seems that Silver has genuine affection for him, and to be accurate Jim also returns the affection.
That said, there is something ferocious about Silver. What makes this even more pronounced is that he only has one leg. In other words, it takes a great deal of effort to overcome his physical handicap.
Now here are a few examples of his ferocious nature. In one case, Silver is talking to a few men about things he has seen at sea. The implication is that he would kill those who thought of crossing him.
‘Why, how many tall ships, think ye, now, have I seen laid aboard? And how many brisk lads drying in the sun at Execution Dock?’ cried Silver. ‘And all for this same hurry and hurry and hurry. You hear me? I seen a thing or two at sea, I have. If you would on’y lay your course, and a p’int to windward, you would ride in carriages, you would. But not you! I know you. You’ll have your mouthful of rum tomorrow, and go hang.’
In another case, a man does cross him. After yelling at him, Silver acts. Here is what the text says:
Silver, agile as a monkey even without leg or crutch, was on the top of him next moment and had twice buried his knife up to the hilt in that defenceless body. From my place of ambush, I could hear him pant aloud as he struck the blows.