But it seems more likely that "indignant" was original, and that a scribe thought Jesus' response was inappropriate. After all, wasn't Jesus a loving person?
(1) "Indignant" is the much more "difficult" reading, since it is more likely that a scribe would have changed "indignant" to "compassionate."
(2) It is clear from preceding verses that Jesus was already well-known as a generous healer (v 34), and the leper's question is therefore a bit odd and perhaps off-putting since people knew he was willing.
(3) In Mark 3:5, Jesus did respond in anger (μετ᾽ ὀργῆς), but this time it was with the Jewish leaders, a common object of his anger (cf. Matt 23).
(4) When the disciples did not allow children to come to him, Jesus became "indignant" (ἀγανακτέω) and "said to them, 'Let the little children come to me, and do not hinder them, for the kingdom of God belongs to such as these'" (Mark 10:14). ἀγανακτέω has a range of meaning, to "be indignant against what is assumed to be wrong, be aroused, indignant, angry" (BDAG).
(5) There are no variants for Mark 3:5 and 10:14, so it can be argued that sometimes Jesus did respond somewhat in anger (frustration?), as perhaps is illustrated in his cleansing of the temple, and Paul later says that proper anger is acceptable and perhaps even appropriate (Eph 4:26).
Does this change our understanding of who Jesus was? Was he, in essence, an angry man? Of course not. A response of anger, whether it be directed at questioning his willingness to heal or directed toward the effects of sin on his good creation, does not make him "an angry man." Certainly a compassionate person, once in a while, can respond in anger and yet not be characterized as "an angry man." Sometimes anger is the right respond to a bad situation, and in this verse he did respond compassionately by healing the leper.