Certainly the Amora'im acted with the highest standards of honesty and morality. Regarding the incident in the Gemara, the Yerushalmi (Shabbos 14:4) makes it clear that the non-Jewish doctor was a charlatan, who depravedly took advantage of sick people in order to extort huge sums of money from them in return for giving them the simplest remedies (such as an aspirin). Many of the sick and suffering fell prey to her dishonest and unethical schemes. Rebbi Yochanan risked sacrificing his own reputation in order to stop her corruption (let her earn a living honestly, and not by taking advantage of sick people).

The verse (Shmuel II 22:27) says that "with the sly, act slyly." When the Gemara (Megilah 13b) asks, "Is it permitted for a Tzadik to act with deceitfulness?" it cites this verse as the allowance for Yakov Avinu's act of making code-signals with Rachel in order that Lavan not deceive him (even though the practice of making codes itself was a form of trickery).

All the best,

Y. Shaw