The merciful man doeth good to his own soul - Every gracious disposition is increased while a man is exercised in showing mercy. No man can show an act of disinterested mercy without benefiting his own soul, by improving his moral feeling.
But he that is cruel troubleth his own flesh - We seldom see a peevish, fretful, vindictive man either in good health, or good plight of body. I have often heard it observed of such, "He frets his flesh off his bones."

The merciful man (k) doeth good to his own soul: but [he that is] cruel troubleth his own flesh.
(k) Rewards both himself and others.

The merciful man doeth good to his own soul,.... Or "to himself": a man of mercy or grace, a liberal bountiful man, he comfortably enjoys what God has given him, Ecclesiastes 5:18; and he does good to others with it, and thereby does good to himself also; as well as he is solicitous in a spiritual sense for the good and welfare of his immortal soul;
but he that is cruel troubleth his own flesh; a sordid avaricious man withholds from himself that which is meet, will not allow himself the necessaries of life, nor will he provide that which is fit and convenient for his family; he hides himself from his own flesh, and will not communicate to the wants of his nearest friends and relations, and shuts up his bowels of compassion against his own brother; all which may be called a troubling his own flesh; see Isaiah 58:7. R. Levi Ben Gersom interprets this of such who place religion in afflicting and macerating the body by fasting, which the law does not require; and it may fitly be applied to the Papists, who do this by penances and fastings, and whippings and scourgings; and which the apostle calls a neglecting of the body, not in any honour to the satisfying of the flesh, Colossians 2:23.

A cruel, froward, ill-natured man, is vexatious to those that are, and should be to him as his own flesh, and punishes himself.

merciful--kind to others; opposed to cruel. Such benefit themselves by doing good to others (compare Proverbs 24:5), while the cruel injure themselves as well as others.
flesh--that is, his body, by penuriousness (Colossians 2:23).

Three proverbs regarding benevolence:
17 The benevolent man doeth good to his own soul,
And the violent man brings trouble on his own flesh.
Many interpreters reverse the relation of subject and predicate (Targ. only in 17b, after the phrase ודמוביד, for which the Syr. has only ומובד): qui sibi ipsi benefacit, is quidem erga alios quoque benignus praesumitur, quum caritas ordinata a se ipsa incipiat; qui vero carnem suam male habet, est crudelis erga alios (Michaelis). But this cannot be established; for certainly it occurs that whoever does good to himself does good also to others, and that whoever is hard against himself also judges and treats others harshly; but in by far the greatest number of cases the fact is this, that he who does not deny anything to himself is in relation to others an egoist, and this is not a "benevolent man;" and, on the contrary, that he who denies to himself lawful enjoyments is in relation to others capable of self-denial and self-sacrifice, and thus is the contrast of a "violent man." The word of Sirach, 14:5, ὁ πονηρὸς ἑαυτῷ τίνα ἀγαθὸς ἔσται, to which Bertheau appeals, alludes to the niggard, and it is true indeed that this עכר שׁארו, but not every עכר שׁארו, is a niggard. Thus the "benevolent man" and the "violent man" will be the two subject conceptions, and as it is said of the benevolent (חסר as e.g., Hosea 6:6, of a more restricted sense, as Isaiah 57:1) that he does good (גּמל, viz., טוב, Proverbs 31:12), so of the violent (unmerciful) (אכזרי as Proverbs 12:20; Jeremiah 6:23; Jeremiah 50:42) that he brings evil on his own flesh (lxx αὐτοῦ σῶμα); for שׁארו as a parallel word to נפשׁו (cf. p. 141) signifies not blood-relations (Symm., Jerome, Luther, and Grotius), but it has here, as at Micah 3:2, its nearest signification, from which it then comes to signify those who are of our flesh and blood. But for that reason the meaning of the poet cannot be that given by Elster: "he who exercises benevolence toward others creates within himself a determination which penetrates his whole being with generous and fruitful warmth, as on the other hand the feeling of hatred deprives the heart of him who cherishes it of the true fountain of life." If this were meant, then soul and spirit, not soul and flesh, would stand in parallelism. The weal and woe refers thus to the divine retribution which requites the conduct of a man toward his neighbours, according to its character, with reward or punishment (Hitzig, Zckler).