I have also seen photos of PLA sodiers being killed. However, in the context of a government mobilising hundreds of thousands of armed troops to suppress a protest by unarmed civilians (I have not seen any evidence that the demonstrators were armed, at least not with guns, depends if you call pots and pans or perhaps anything you can find at home weapons), shooting their way into Tiananmen (many eyewitness accounts seem to concur that most deaths were on the way to the square, not inside the square. The Beijing people were trying to block the army trucks and tanks from entering the city and the square, and the army shot them with automatic rifles), the people were rightfully angry. So when they got hold a a few PLA sodiers they killed them. I'm not saying that the people were right in killing them. But can you really compare the level of violence, and balance of (fire) power, between the PLA and the civilians in Beijing?
Yes, many Hong Kong demonstrators demand an apology from the Mainland Government, because they think the Mainland Government was wrong to open fire on their own people. Why is the Mainland Government completely silent now, not even trying to defend itself? Because deep down they know that's what they did: they deployed the army to kill their own people.
Yes some Hong Kong people are demanding democracy, because they believe that only when the power is with the people that there can be real check-and-balance agasint corruption, social injustice, and regime brutality. Sure you can cite a hundred examples of how a democratic governemnt can also be brutal to its own people, but do you really believe that it has a higher chance of doing that than a dictatorship? Or perhaps you would say that a western style democracy is not suitable to China, at least it cannot be implemented overnight. I think only really naive people would think that the CCP may collapse overnight and everything would be great immediately. The road to democracy will be long and winding, we know, even with the best intentions from the government, which is not even likely. However, it is important to keep the demand, the ultimate goal, alive. If people stop asking for it, even in such a peaceful way as a candlight vigil, there will be absolutely no incentive for those in power to even slightly release their power, especially as China becomes richer and richer, and the material benefit that comes with such power become more and more attractive.