Actually, most of the arbok you’ll see on TV are either:
1. Trained to be that aggressive (for entertainment purposes or because battle-ready arbok will usually be aggressive for fairly apparent reasons).
2. Wild (and thus aggressive to defend itself).
However, tamed arbok will tend to be as gentle as their trainers want them to be, so if you raise your ekans to be gentle and sweet, you’ll end up with a gentle and sweet arbok. The only thing to note is that it is true that a pokémon’s personality changes slightly after it evolves, but it’s not in the way one might think. It’s not a dramatic or otherwise drastic change; rather, it’s a transition into maturity. If your ekans is playful, he may be more reserved after evolving. Alternatively, if your ekans is shy, he may be more energetic or outgoing as an “adult.” In other words, he won’t be dangerous, but it wouldn’t hurt to keep an eye out for shifts towards a more mature personality.