Addiction to cheap resources prevented politically sustainable energy sector development. On top of that, it's likely that corruption was involved. Which is cancer that tends to cause more corruption in semi-related parts of the system. Once politicians sell out, it's much easier to bend them again. Be it pure greed or blackmailing.
Put another way, if it weren't for USA's hard stance, many large EU economies would have few misgivings about continuing to do business with Russia. And even hardliners like Poland AFAIK are still buying Russian gas through Germany, as do several others including Ukraine.
The reason they're doing that is because they need it, it's there and it's good for keeping their economies and societies in functioning order.
Was there corruption? You bet, former USSR states were and are drowning in it and the West was certainly not spared either. Doesn't change the fact that they need resources though and there's not a lot of alternative suppliers.
Trade with Russia was obviously positive for the EU, as can be noticed from the economic damage and energy availability crisis.