Blazing Spear is a good skill, but it can be a little wonky trying to get it to work, especially when it's lagging. It's good for helping a tank with resources, and is good for damage. It's just wonky at times.
As for the execute, it starts at 50% health. It's a great skill and it puts minor sorcery in the group. As for PvE, with a proper heavy attack build, your normal rotation with your heavy attacks while weaving your skills can out dps the execute by itself. It would depend on the player and how well they do their rotation. If the rotation needs work, the execute might be the better option.