The author of that article is implying that not everything about Li Simin is good. He benefited from his murderous scheme to kill off both his brothers, to get what he wanted, even though this resulted in him being the emperor and being a very good one.
If you are game to read more about ethical theories, you can try this: https://en.wikipedia.org/wiki/Consequentialism
Basically, what he did in the coup could be dangerous, if we say that it is justifiable. No one could have predicted that he would turn out to be a great emperor. And even if he did, the means to do it will still be wrong if it is against one's virtues (e.g., killing someone is wrong). So, you could well question his virtue, when he was planning to have both his brothers killed. Could he have spared them and put them in permanent imprisonment? I suppose that will be rather troublesome. During those times, killing off one's political opponents was always a fair game. A game of cat and mouse, like you like to put it. Maybe Li Jiancheng just was not as smart or as cruel as Li Si Min?
In the end, this is always going to be a highly personal issue, what is right or wrong. But, by today's standard, killing someone is always going to be wrong, regardless of what is one's intention or the outcome of that killing. For example, a policeman killing a known criminal in cold blood. The criminal must instead be brought to justice legally.
