Translation:The warrior has defected.
Mostly it would be redundant. The -ta' tells us that he accomplished it. If he accomplished it, then he must of have been doing it on purpose. You can say "he accomplished on purpose" if you want, but it doesn't add any information. The real question is just how cheHta' SuvwI' differs from chIch cheHpu' SuvwI'.
It's more of a philosophical one. The -ta' suffix means that it was a deliberate act, which is what chIch means, but I'm not sure I'd use -ta' if it were a spontaneous action. The -ta' is a 'set out to do it, and did it' action. Remember that -pu' can always be used for any completed action, so using -ta' likely has some significance.
So lets say that there has been a battle, and a number of warriors have been captured. Their captors tell them that they will have one chance to defect, at roll call tomorrow morning. Anyone who correctly salutes the commander will be accepted into their ranks, and those who do not will be executed. PW#1 decides that night that he wants to live and will defect. The next day when the enemy commander strides out to supervise roll call, PW#1 snaps a sharp salute. PW#2 was going to hold fast and face death, but when he sees his comrade do it, he decides to put dishonour before death and also defect. PW#3 is just cold and hungry but sees his two comrades saluting and hastily follows suit. All three are led away to their former enemy's mess hall, while the remaining prisoners are shot where they stand.
cheHta' PW#1. chIch cheHpu' PW#2. bong cheHpu' PW#3.
"Fighter" is accepted on the variations of this sentence. The word SuvwI' can be translated as "fighter" and we have tried to always include that in the alternate translations, but in general, the preferred translation of SuvwI' is almost always "warrior".
Since both are accepted on this sentence your error was actually a different one. You have not included a translation for the -ta' suffix. The Klingon perfective suffixes do not match perfectly to the English perfect tenses and there are some situations where you could translate a Klingon perfective suffix with the English simple past tense. However, in this course we ask that you always translate the Klingon perfective suffixes by using the English perfect tenses (in other words, by including "has", "had", or "will have"). You were graded wrong for not including the word "has" and you were shown the version marked as the "Best" translation which also uses the word "warrior", even though your use of "fighter" was acceptable.