There is a clear halacha that it is a Torah prohibition to remind a baal teshuva of his past (Shulchan Aruch C.M. 428:4) - this is from the following gemora.
Bava Metzia (58b):If a man is a penitent, one must not say to him, ‘Remember your former deeds.’ If he is the son of proselytes he must not be taunted with, ‘Remember the deeds of your ancestors. If he is a proselyte and comes to study the Torah, one must not say to him, ‘Shall the mouth that ate unclean and forbidden food,14 abominable and creeping things, come to study the Torah which was uttered by the mouth of Omnipotence!’ If he is visited by suffering, afflicted with disease, or has buried his children, one must not speak to him as his companions spoke to Job, is not thy fear [of God] thy confidence, And thy hope the integrity of thy ways? Remember, I pray thee, who ever perished, being innocent?1
My question is how to explain Rav Yochanon's behavior with Reish Lakish which resulted in them both dying - as is described in the following gemora?
Bava Metzia (84a): One day R. Johanan was bathing in the Jordan, when Resh Lakish saw him and leapt into the Jordan after him. Said he [R. Johanan] to him, ‘Your strength should be for the Torah.’13 — ‘Your beauty,’ he replied, ‘should be for women.’ ‘If you will repent,’ said he, ‘I will give you my sister [in marriage], who is more beautiful than I.’ He undertook [to repent]; then he wished to return and collect his weapons, but could not.14 Subsequently, [R. Johanan] taught him Bible and Mishnah, and made him into a great man. Now, one day there was a dispute in the schoolhouse [with respect to the following. Viz.,] a sword, knife, dagger, spear, hand-saw and a scythe — at what stage [of their manufacture] can they become unclean? When their manufacture is finished.15 And when is their manufacture finished? — R. Johanan ruled: When they are tempered in a furnace. Resh Lakish maintained: When they have been furbished in water. Said he to him: ‘A robber understands his trade.’16 Said he to him, ‘And wherewith have you benefited me: there [as a robber] I was called Master, and here I am called Master.’17 ‘By bringing you under the wings of the Shechinah,’ he retorted. R. Johanan therefore felt himself deeply hurt,18 [as a result of which] Resh Lakish fell ill. His sister [sc. R. Johanan's, the wife of Resh Lakish] came and wept before him: ‘Forgive him19 for the sake of my son,’ she pleaded. He replied: ‘Leave thy fatherless children. I will preserve them alive.’20 ‘For the sake of my widowhood then!’ ‘And let thy widows trust in me,’21 he assured her. Resh Lakish died, and R. Johanan was plunged into deep grief. Said the Rabbis, ‘Who shall go to ease his mind? Let R. Eleazar b. Pedath go, whose disquisitions are very subtle.’ So he went and sat before him; and on every dictum uttered by R. Johanan he observed: ‘There is a Baraitha which Supports you.’ ‘Are you as the son of Lakisha?’22 he complained: ‘when I stated a law, the son of Lakisha used to raise twenty-four objections, to which I gave twenty-four answers, which consequently led to a fuller comprehension of the law; whilst you say, "A Baraitha has been taught which supports you:" do I not know myself that my dicta are right?’ Thus he went on rending his garments and weeping, ‘Where are you, O son of Lakisha, where are you, O son of Lakisha;’ and he cried thus until his mind was turned. Thereupon the Rabbis prayed for him, and he died.