hi, just a few points:
1. the relative should be put in the case required for the clause in which it stands (see smyth s2501). In your relative clause it needs to be nominative because it represents the subject of the 3p pl verb. by putting it in the accusative, it looks as if you are attracting the case of the relative from nominative (i.e. from its required case within its phrase) into the case of object of the main clause ἀποκτείνωμεν (i.e. into the accusative). however it’s very rare for a nominative relative to be attracted into another case (see smyth s2523), and ὅστις is only attracted in the construction οὐδεὶς ὅστις οὐ (see smyth ss2524 and 2534). so here you should put the relative in the nominative, not the accusative.
2. the 3p pl verb should be formed from ἐπιβουλεύω, not βουλεύω.
3. as ἡμῖν is in a dependent clause I would put it in the indirect reflexive form ἡμῖν αὐτοῖς, as it refers back to the subject of the main clause ἀποκτείνωμεν (see smyth ss 329 and 1225). see isocrates’ panegyricus s106 for an e.g. of this: τοιαῦτ᾽ ἔχοντες ταῖς ὀλιγαρχίαις ἐπιτιμᾶν καὶ πλείω τούτων, τὴν αὐτὴν πολιτείαν ἥνπερ
παρ᾽ ἡμῖν αὐτοῖς
καὶ παρὰ τοῖς ἄλλοις κατεστήσαμεν.
4. after πάντας, I would have expected a simple relative (such as ὅσοι) referring to the definite persons referred to by πάντας, as in thucydides s4.57.4: καὶ τούτους μὲν οἱ Ἀθηναῖοι ἐβουλεύσαντο καταθέσθαι ἐς τὰς νήσους, καὶ τοὺς ἄλλους Κυθηρίους οἰκοῦντας τὴν ἑαυτῶν φόρον τέσσαρα τάλαντα φέρειν, Αἰγινήτας δὲ ἀποκτεῖναι πάντας ὅσοι
ἑάλωσαν διὰ τὴν προτέραν αἰεί ποτε ἔχθραν, Τάνταλον δὲ παρὰ τοὺς ἄλλους τοὺς ἐν τῇ νήσῳ Λακεδαιμονίους καταδῆσαι.
5. just a question: in your translation (C), are you treating ἀποκτείνωμεν as aorist, and putting the dependent verb in the optative to make an indefinite construction in historic sequence, of the type described in s21(2)(b) on page 14 of sidgwick’s intro to greek prose comp (available on this site)?